Proline X

Performance Optimized Down Fire Sealed 12" Enclosure | PowerBass XL-1240T

Q: Will this enclosure fit any 12 inch subwoofer?

No. The baffle is cut direct fit at 281 mm for the PowerBass XL-1240T and the airspace is built to that driver. The XL-T is a shallow mount driver with a 3.8 inch mounting depth, and a conventional 12 needs both a deeper cabinet and more air.Even where another driver dropped into the hole, it would be loading into an airspace engineered around a Qts of 0.526, and the alignment printed on this page would no longer describe what you own.

Q: PowerBass says 0.40 cubic feet. Is this cabinet the same size?

Almost, and the difference is worth understanding. PowerBass quotes 0.40 cubic feet as internal airspace that INCLUDES the driver's displacement, so their figure is gross. We publish net everywhere, meaning the air the cone actually loads against after the driver's 0.1260 cubic feet is taken out.This cabinet is 0.40 net, which is 0.5260 gross. That puts it slightly on the larger side of their optimum and comfortably inside the 0.30 to 0.85 cubic foot range they list as acceptable.

Q: What amplifier power and impedance?

The XL-1240T is rated 400 watts RMS and 800 peak. It is a single 4 ohm voice coil.A single driver simply presents 4 ohms so there is no low impedance trap to fall into on this driver. Target 400 watts RMS and set gains properly rather than to a number.

Q: How tall is this once the standoffs are on?

The cabinet is 5.30 inches tall measured in the down fire position, excluding standoffs. They hold 1.75 inches under the baffle, so plan around a finished stack of roughly 7.05 inches.The footprint is 24.89 inches wide by 15 deep. That is narrower than the standard 30 inch down fire platform, because this driver needs so little air that the full width would overshoot the target.

Q: Does the pole vent need clearance under the cabinet?

No. The XL-T uses a vented rear motor, so the pole vent can be obstructed without harming the driver. That is what allows this cabinet to be built without the rear clearance allowance most drivers require, and it is a large part of why a shallow driver suits a down fire cabinet at all.The standoffs are there to give the cone room to breathe in front of the baffle, which faces the floor, not to relieve the pole vent.

Q: What is the expected low-frequency extension?

Anechoic F3 lands at 53.6 Hz with system Qtc 1.145 and fc 71.9 Hz, calculated from PowerBass's published parameters at the 0.40 cubic feet net this cabinet holds.Those figures are anechoic and exclude cabin gain. In a car the transfer function of the cabin adds real output below 50 Hz, so in-vehicle response runs deeper. A shallow mount driver in a small sealed box is a packaging decision, and the alignment reflects that honestly rather than promising otherwise.

Q: Do I need to add polyfill or damping?

No. Polyfill is already installed as a light wall lining to control internal reflections, and the quantity is chosen so it does not change the acoustic volume.Do not add more. Packing a sealed box with fill makes it behave as though it were larger, which on a cabinet this small would move the alignment noticeably. PowerBass also warns against letting fill reach the vented pole piece.

Q: Is the PowerBass XL-1240T included?

No. This is an enclosure only. You receive the cabinet finished in plush black carpet, with threaded inserts, stainless machine screws, a side-mounted pre-wired terminal cup, the standoffs and the polyfill lining already in place.The driver is purchased separately. We do not stock PowerBass drivers.

SKU: PO 12-S DF XLT

QUICK SPECS

Alignment: 0.40 cu ft net. Qtc 1.145, fc 71.9 Hz, F3 53.6 Hz
Driver: PowerBass XL-1240T, 400 W RMS, 7.5 mm Xmax, 3.8 in mounting depth, single 4 ohm
Rear Venting: Vented rear motor, pole vent may be fully obstructed
Standoffs: Included, 1.75 in, uninstalled. Installed height near 7.05 in
Cabinet: 24.89"W x 15"D x 5.30"H in down fire position, excludes standoffs

Shipping Included No freight charges at checkout. What you see is what you pay.

Built in Tennessee CNC cut from 3/4" Langboard Elite MDF. V-groove and dado joinery.

Advanced Terminal Cup Built in-house. ABS/Carbon Fiber. Stainless steel hardware. Copper ring terminals.

$ 249.99

In Stock and Ready to Ship

Financing available. Choose Shop Pay Installments at checkout.

Performance Optimized Down Fire Sealed 12" Enclosure | PowerBass XL-1240T

$ 249.99

Enclosure Type Sealed

External Dimensions 24.89" W x 15" D x 5.30" H (measured in down fire position, excludes standoffs)

Net Volume 0.40cu ft

Tuning Frequency N/A

Cutout Diameter Direct Fit

Front Panel Thickness 1.5"

Max Mounting Depth Direct Fit

INSTALLER'S TAKE

Watch the volume convention on this brand. PowerBass quotes 0.40 cu ft and that is GROSS, displacement included. This cabinet is 0.40 NET, which is 0.5260 gross. If a customer says the box looks big against the sheet, that is why, and it is inside their 0.30 to 0.85 acceptable range.

Measure the installed height, not the cabinet height. 5.30 tall plus 1.75 of standoff puts you at 7.05 inches of stack. Footprint is 24.89 by 15.

Rear vented motor, so the pole vent can be blocked. That is why this cabinet has no rear clearance allowance and why it comes out as short as it does.

Single 4 ohm coil. One driver is simply 4 ohms. Nothing to get wrong.

Qtc 1.145 at 0.40 net. This is a shallow driver in a small box and it reads that way: tight rather than deep. Set the expectation before the sale.

Product Details

Proline X Performance Optimized Down Fire 12" Sealed

Cut and tuned for the PowerBass XL-1240T

V-Groove and DadoLangboard Elite MDF0.40 cu ft NetStandoffs IncludedShallow Mount

0.40cu ft net

1.145system Qtc sealed

53.6 HzF3 rolloff

5.30 incabinet height, excludes standoffs

Performance, Performance Optimized, and Micro

Proline X builds three tiers of Down Fire enclosure. The difference is not finish or materials, it is whether the cabinet was designed for a category or for a driver.

	Performance	Performance Optimized	Micro Series
Fitment	Universal. Accepts any subwoofer that clears the published cutout and mounting depth.	Woofer specific. Cut for one named subwoofer, with cutout, mounting depth, magnet clearance and displacement drawn in CAD from that driver.	Woofer specific. Cut for one named subwoofer.
Footprint	Sized to the driver diameter.	A shared 30 by 15 inch platform on most builds. This one runs narrower at 24.89 inches, because the XL-T needs so little air that the standard width would overshoot the target before the cabinet was even tall enough to clear the motor.	Compact per driver footprint.
Front baffle	Single, 3/4 inch.	Double, 1.5 inch, outer layer machined as a flush recess.	Double, 1.5 inch.
Driver mounting	Solid flange. You drill your own mounting holes.	Threaded inserts, stainless machine screws included.	Threaded inserts, stainless machine screws included.
Joinery	V-groove and dado, CNC cut.	V-groove and dado, CNC cut, perimeter cross brace.	Stack fab, dowel reinforced.
Published alignment	Modeled Qtc profiles across a range of drivers. No single response curve.	A real predicted Qtc and F3 for that exact driver, calculated from its own Thiele-Small parameters.	Target alignment described in the listing.
Carpet	Gray standard, black optional.	Plush black.	Plush black.
Warranty	1 year limited.	1 year limited.	2 years.

Feet ship with every tier, uninstalled on purpose. Mount them where your floor wants them for a down fire install, or leave them off and run the cabinet front firing or up firing.

Built Around the PowerBass XL-1240T

The baffle is bored to the XL-1240T's 281 mm cutout and the airspace is built to 0.40 cubic feet net. This is a shallow mount driver: 3.8 inch mounting depth, 7.5 mm of one-way excursion on a 2 in voice coil, 400 watts RMS and 800 peak, rated 33 Hz to 450 Hz.

PowerBass names 0.40 cubic feet as the optimum sealed volume and lists 0.30 to 0.85 as the acceptable range. They do not recommend a ported enclosure for this driver at all.

Net Volume, Not Gross

We build this cabinet at 0.40 cubic feet net: true internal airspace after panel thickness, bracing and the driver's own 0.1260 cubic feet of displacement are accounted for. Gross is 0.5260 per chamber.

Worth knowing when you compare figures. PowerBass quotes its 0.40 cubic foot recommendation as internal airspace that includes the driver's displacement, so their number is gross where ours is net. Building 0.40 net rather than 0.40 gross puts this cabinet slightly on the larger side of their recommendation, well inside the 0.30 to 0.85 range they publish.

What the Alignment Actually Is

Calculated from PowerBass's own published Thiele-Small parameters: Fs 33 Hz, Qts 0.526, Vas 42.37 litres. At 0.40 cubic feet net that gives system Qtc 1.145, fc 71.9 Hz and F3 53.6 Hz anechoic. That is a deliberately small-box alignment with real lift at resonance.

A shallow driver in a small box behaves like a shallow driver in a small box. The XL-T trades cone area and excursion for a 3.8 inch mounting depth, and the sealed alignment that suits it is a tight one rather than a maximally flat one. Those figures are anechoic and exclude cabin gain, which adds real output below 50 Hz in a car.

Down Fire Construction

Cut with V-groove and dado joinery on our ShopSabre routers from Langboard Elite 3/4 inch MDF. Every panel joint is a machined interlocking groove that mechanically locks before adhesive is applied, which keeps the cabinet airtight under sustained pressure cycling. Corners are square on Down Fire cabinets.

The cabinet measures 24.89 inches wide by 15 inches deep by 5.30 inches tall, measured in the down fire position and excluding standoffs. The standoffs hold 1.75 inches under the baffle, so the finished stack sits near 7.05 inches.

Double Baffle, Flush Recess

The front baffle is 1.5 inches, two bonded 3/4 inch sheets, with the outer layer machined as a flush recess so the driver bolts to the inner layer.

Side-Mounted Terminal Cup

Proline X ABS and carbon fiber composite cup with stainless hardware and copper ring terminals, pre-wired in 12 gauge OFC.

Standoffs Included

Custom fit standoffs ship with every enclosure at 1.75 inches, uninstalled so you can place them to suit your floor.

Rear Vented Motor, So the Floor Is Not a Problem

The XL-T uses a vented rear motor, so the pole vent can be obstructed without harming the driver. On a down fire cabinet that matters more than it sounds: the motor sits inside the enclosure firing at the top panel, and a driver that needs its pole vent kept clear forces either a taller box or a compromised one.

This cabinet clears 0.75 inches above the motor at 3.05 inches of internal height. The feet ship uninstalled, so front firing or up firing stays available to you at install time without changing anything about the enclosure.

Power and Wiring

The XL-1240T is a single 4 ohm voice coil, so it presents 4 ohms and there is no wiring decision to make. Target 400 watts RMS.

The enclosure carries a 1 year limited warranty. Bring your own XL-1240T; the driver is purchased separately.

Cut for the Driver, Not the Catalog

Built one at a time in Tullahoma, Tennessee, with standoffs and a 1 year limited warranty. Questions on fitment or amplifier matching, call or text 931-563-5151.

Specifications

Specification	Value
Series	Proline X Performance Optimized Down Fire
Compatible Driver	PowerBass XL-1240T, quantity 1. Direct fit
Configuration	Single 12 inch shallow mount, sealed down fire
Mounting Position	Trunk or cargo floor, baffle facing down on standoffs
Firing Direction	Downward. Feet ship uninstalled, so front or up firing is also possible
External Width	24.89 in
External Depth	15 in
External Height	5.30 in in the down fire position, excludes standoffs
Installed Height	7.05 in including the 1.75 in standoff
Internal Volume, Gross	0.5260 cu ft
Internal Volume, Net Acoustic	0.40 cu ft after 0.1260 displacement
PowerBass Published Target	0.40 cu ft optimum, 0.30 to 0.85 acceptable. Their figures are GROSS and include driver displacement
Alignment	Qtc 1.145, fc 71.9 Hz, F3 53.6 Hz anechoic
Woofer Opening	11.0000 in (281 mm) direct fit
Driver Mounting Depth	3.8 in required, shallow mount
Driver Motor	Rear vented, pole vent may be fully obstructed
Driver Power Handling	400 W RMS, 800 W peak
Driver Voice Coil	Single 4 ohm, 2 in
Wiring	4 ohms, no wiring choice
Material	Langboard Elite MDF
Construction	CNC cut, V-groove and dado joinery, square corners
Panel Thickness	0.75 in
Front Baffle	1.5 in double baffle, outer layer machined as a flush recess
Internal Bracing	Windowed U-brace, dadoed to front, top and back
Driver Mounting	Threaded inserts, stainless machine screws included
Terminal Cups	One Proline X ABS and carbon fiber composite, side mounted, pre wired 12 gauge OFC
Standoffs	Included, 1.75 in, shipped uninstalled
Damping	Light polyfill wall lining, does not alter acoustic volume
Carpet	Plush black
Weight	26 lb
Warranty	1 Year Limited
Built In	Tullahoma, Tennessee

Performance Graphs

Net airspace

0.40

cu ft per chamber

System Qtc

1.15

sealed

Half power

53.6

Hz · F3

Xmax at

425

W RMS · no filter

With 25 Hz subsonic

425

W RMS · +0%

Amplifier

400

W RMS recommended

This enclosure holds 0.40 cu ft net per chamber. In it the PowerBass XL-1240T runs a system Qtc of 1.15 with half power at 53.6 Hz, and the cone reaches its 7.5 mm travel limit at 425 W, above the driver’s 400 W thermal rating, so heat stops you before travel does. A 25 Hz subsonic filter adds little here: 425 W against 425 W, 0 percent. The box already resonates at 72 Hz, so its own rolloff has removed most of what the filter would have stopped.

01Is it a sealed driver?

Efficiency bandwidth product places the motor before anything else is decided.

EBP = Fs ÷ Qes = 55.3

XL-1240T · EBP 55.3

At 55.3 the XL-1240T sits in the transition band, not the sealed band. That is not a mismatch: a transition motor works in either alignment, and sealed buys a smaller box and a gentler roll-off at the cost of some low-end output. Stage 02 shows what this airspace actually does with it.

02What the box does to it

0.40 cu ft against a 42.4 L Vas.

α = Vas ÷ Vb = 3.74 · Fc = Fs√(α+1) = 72 Hz · Qtc = 1.15

XL-1240T · Qtc 1.15, Fc 72 Hz, F3 53.6 Hz

The box lifts resonance from 33.0 Hz free-air to 72 Hz and half power lands at 53.6 Hz, anechoic, before any cabin gain.

03The watts that reach Xmax (in this enclosure)

Travel scales with the square root of power, so there is one wattage that puts the cone exactly on 7.5 mm, and in this alignment a subsonic filter barely moves it.

P_Xmax = 425 W unfiltered → 425 W with a 25 Hz subsonic

XL-1240T · 425 W, no filterXL-1240T · 425 W with subsonic at 25 Hz, 24 dB/octXmax 7.5 mm one-way

Every trace touches the limit and no further, because each is drawn at the exact power that puts that coil on 7.5 mm. Unfiltered that is 425 W, reached first at 56 Hz. It sits above the 400 W the driver can take thermally, so the voice coil gives up before the suspension does. The dashed traces are the same driver behind a subsonic filter at 25 Hz, 24 dB/oct, and they sit almost on top of the solid ones: 425 W against 425 W. At 72 Hz the filter sits well below resonance, where the sealed alignment has already rolled the cone off at 12 dB/oct, so there is little wasted travel left for it to recover. Set it anyway to keep road rumble out of the cone, but do not size the amplifier around it.

Param	Value	Unit	Note
Fs	33.00	Hz	free-air resonance
Qts	0.526		XL-1240T
Vas	42.37	L	equivalent compliance
Sd	510.7	cm²	effective piston area
Xmax	7.5	mm	one-way linear travel
Pe	400	W	continuous RMS
SPL	85.9	dB	1 W / 1 m, derived from T/S

Driver parameters as published by powerbassusa.com. Curves computed against this enclosure’s actual net airspace.

Materials

Material

Built From a Spec Sheet, Not a Sales Sheet

Every Proline X Performance Series enclosure is CNC-cut from Langboard Elite MDF, 3/4" (18mm) panels produced to published mill specifications. Most enclosure manufacturers source generic MDF without published data. We can tell you the internal bond rating, the modulus of elasticity, the modulus of rupture, and the face and edge screw holding strength for every panel in this enclosure, because we chose the material based on those numbers, not on price or availability.

Langboard Elite is produced in the United States. The specifications below are pulled directly from Langboard's published mill data and are verifiable. This is the same panel stock used in our full Proline X lineup.

Specifications

Langboard Elite MDF: Published Mill Data

Property	Value	Why It Matters
Panel Stock	Langboard Elite MDF	Named, verifiable panel source, not generic "high-density MDF"
Thickness	3/4" (18mm)	Industry standard for subwoofer enclosures; panels below this flex under pressure
Density	48.5 lbs/ft³	Higher density = less porosity, better screw retention, reduced resonance
Internal Bond	200 psi	Measures resistance to delamination under sustained internal pressure. The most enclosure-relevant spec on this sheet
Modulus of Rupture (MOR)	4,200 psi	Bending strength before the panel breaks, relevant for larger unsupported baffle spans
Modulus of Elasticity (MOE)	410,000 psi	Stiffness under load; higher MOE means less panel flex under bass pressure, less resonance
Face Screw Holding	325 lbs	How firmly the panel surface holds a screw; critical for baffle joint integrity over time
Edge Screw Holding	250 lbs	Panel edge retention, relevant for every joint that isn't face-to-face

Internal Bond

200 psi: The Number That Actually Matters for Enclosures

The internal bond rating measures how well the MDF panel resists delamination under sustained pressure applied perpendicular to the face. In a subwoofer enclosure, every bass transient generates a pressure pulse that pushes outward on every panel simultaneously. A panel that begins to delaminate internally loses stiffness gradually. The symptoms are slow: a box that starts to sound softer, less defined, less punchy over months of use. 200 psi internal bond is the spec that determines how long this enclosure holds its acoustic integrity under daily use at real power levels.

Stiffness

410,000 psi MOE: Less Flex, Less Resonance

The modulus of elasticity measures panel stiffness, its resistance to bending under load. A stiffer panel flexes less under the pressure generated by a high-excursion driver at volume. Panel flex is a source of acoustic coloration: the panel itself begins to radiate sound at its own resonant frequency, adding distortion to the output. 410,000 psi stiffness combined with V-groove and dado joinery means the panels stay rigid and the bracing controls resonance at the structure level, not the panel level. The bass you hear comes from the driver, not the box.

Screw Holding

325 lb Face / 250 lb Edge: Joints That Stay Locked

Screw holding strength determines how firmly the panel grips a fastener at the face and at the edge. This matters most at the baffle joint, where the front panel meets the side panels and where the driver is mounted. A panel with low screw holding strength allows fasteners to loosen over time under vibration, leading to air gaps at the joint and loss of acoustic seal. 325 lb face screw holding strength means the baffle hardware stays put. Combined with dado joinery and adhesive, the front panel on a Proline X Performance Series enclosure is not going anywhere.

Sourcing

Named Panel Stock: Not "Premium MDF"

The phrase "premium MDF" or "high-density MDF" appears on marketing copy for enclosures across every price point, from $40 to $400. It is not a specification. It is a description with no defined standard. Langboard Elite is a named, sourced, mill-documented product with published data. We name it in our copy because naming it holds us accountable to the spec. You can look up Langboard Elite independently, verify the numbers on this page, and confirm you are getting what we say you are getting. That is the point.

Why Proline X?

Most enclosures are built to a price point. Proline X is built to a spec. Every box starts with published-data MDF, gets CNC-cut to exact internal volume, and is assembled by hand in our Tennessee facility. The difference shows up in the measurements, and in how the box holds up two years into daily use.

CNC-Cut to Spec

Panels are machine-cut on ShopSabre CNC routers and joined with V-groove and dado joinery. Consistent geometry, no hand-cut variance, no gaps at the joints.