Proline X
Performance Optimized Down Fire Wide Sealed 12" Enclosure | Hertz Mille Pro MP 300
SKU: PO 12-S DFW MP
- Alignment: 1.512 cu ft net. MP 300 D4.3 Qtc 0.694, Fc 39.6 Hz, F3 40.4 Hz; MP 300 D2.3 Qtc 0.839, Fc 40.2 Hz, F3 34.8 Hz
- Down Fire Wide: 41"W x 14"D x 8"H in down fire position, lower than the standard 30 x 15 (8.125 in)
- Height Set By: the 6.457 in motor, 0.75 in clearance above it; holds more air than the standard box, so Qtc is lower
- Driver: Hertz MP 300 D2.3 or D4.3, 600W RMS / 1200W peak, 17mm Xmax, 86 dB sensitivity, 6.457" mounting depth, 11.30" cutout
- Build: V-groove and dado joinery, 1.5" double baffle with flush outer recess, windowed U-brace, 12g OFC side-mounted terminal cup
Financing available. Choose Shop Pay Installments at checkout.
Performance Optimized Down Fire Wide Sealed 12" Enclosure | Hertz Mille Pro MP 300
More enclosures cut for Hertz subwoofers: Hertz Subwoofer Boxes
Proline X Performance Optimized Down Fire Wide 12" Sealed
Cut for the Hertz Mille Pro MP 300, D2.3 or D4.3
The Down Fire Wide Format
Down Fire Wide is the same driver in a different shape. The standard Performance Optimized Down Fire cabinet for the Hertz Mille Pro MP 300 is 30 by 15 inches. This one runs 41 inches wide by 14 inches deep: 1 inch shallower front to back and 11 inches longer.
Fourteen inches is not arbitrary. A 12 inch frame needs about 13 inches of baffle to sit on, so 14 is the practical minimum depth for this driver size. The Hertz Mille Pro MP 300 frame measures 12.638 inches across and sits on the 14 inch deep baffle with about 0.68 inches of baffle fore and aft.
Spreading the airspace along 41 inches instead of 30 means the cabinet does not have to climb to find it. This box stands 8 inches tall, against 8.125 for the standard 30 x 15 Hertz Mille Pro MP 300 cabinet, both measured in the down fire position without feet. That is 0.125 inches lower.
Which one to choose. The standard 30 x 15 is the pick when length is limited. This one is the pick when height or depth is the limit and you have 41 inches of run. With the 1.75 inch feet installed this cabinet stands 9.75 inches, so measure for that.
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, and how much of the fitting work is already done for you.
| 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. 24.5 by 13.5 inches on the 10 inch. | Shared 30 by 15 inch platform on most builds. Down Fire Wide variants like this one run long and shallow for under seat fitment. | Compact per driver footprint. 21.5 by 14.5 inches on the 10 inch. |
| Front baffle | Single, 3/4 inch. | Double layer, outer layer machined as a flush recess. 1.5 inch. | Double, 1.5 inch. |
| Driver mounting | Solid flange. You drill your own mounting holes. | 8/32 threaded inserts, stainless machine screws included. | 8/32 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. The orientation is yours to choose.
This page is a Performance Optimized enclosure. The cutout, the cabinet height and the airspace all come from the Hertz Mille Pro MP 300's own published numbers. A Performance box fits your subwoofer. A Performance Optimized box was built for it.
Built Around the Hertz MP 300
The baffle is bored to the MP 300's 287mm cutout, 11.30 inches, and the internal height is set by its 6.457 inch mounting depth. The driver bolts to 8/32 threaded inserts with stainless machine screws included, and at 8.63 kg it is a heavy motor that wants a baffle it can pull against.
Hertz publishes separate Thiele-Small sets for the two coil versions: the MP 300 D4.3 at Fs 24 Hz, Qts 0.42 and 74 liters of Vas, and the MP 300 D2.3 at Fs 23 Hz, Qts 0.48 and 88 liters. Both are genuinely capable foundations for a sealed 12.
The cone is a V-cone profile pressed from paper pulp with mineral powder injection, the technique Hertz uses on the Mille Legend line, so the diaphragm behaves like a piston rather than flexing at the edge. The surround is high-purity IIR butyl rubber. A 65mm four-layer CCAW voice coil sits on a vented TIL-P former, driven by a high-density ferrite motor in a six-spoke aluminum alloy basket vented under the spider. Excursion is 17mm linear with 27mm of mechanical travel before damage.
If you are cross shopping our Hertz cabinets: the Mille Pro MPS series is the shallower line and gets its own cabinets. This page is cut for the Mille Pro MP 300 only.
Net Volume and Alignment
The cabinet holds 1.512 cubic feet net, from 1.601 cubic feet gross after the brace and the driver's 0.08 cubic feet of displacement (estimated, no manufacturer figure).
Height here is set by the driver's mounting depth, not by a volume target. The 6.457 inch motor needs 0.75 inch of clearance above it, so the cabinet can go no lower than 8 inches. At that height the wide footprint holds 1.512 net, more than the 1.10 the standard 30 x 15 box was built to (our solver puts its true net at 1.173). More air lowers Qtc, which is the safe direction: a softer, better damped corner rather than a peakier one.
Computed from Hertz's published Thiele-Small parameters for each coil: MP 300 D4.3: Qtc 0.694, Fc 39.6 Hz, F3 40.4 Hz; MP 300 D2.3: Qtc 0.839, Fc 40.2 Hz, F3 34.8 Hz.
Hertz recommends 28 liters sealed; this cabinet gives about 43, set by the motor rather than chosen. The effect runs the safe way. The D4.3 lands just under maximally flat, controlled and even. The D2.3, with its higher Qts and larger Vas, keeps more lift near the corner and reaches about 5.6 Hz deeper. Firing down into the floor, the sealed rolloff meets rising cabin gain below 60 Hz.
True net, not marketing net. Internal volume is calculated from the machined cavity dimensions and the driver's displacement, not estimated from external dimensions minus wall thickness.
Down Fire Wide Construction
Cut with V-groove and dado joinery on our ShopSabre routers from Langboard Elite 3/4 inch MDF, a dense 48.5 lb/ft³ board with a 200 psi internal bond and 410,000 psi MOE. 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 41 inches wide by 14 inches deep by 8 inches tall, measured in the down fire position. Feet ship with the enclosure but are not installed. With the 1.75 inch feet on, the cabinet stands 9.75 inches, so plan the space around where you intend to mount them.
1.5 Inch Double Baffle
Two 3/4 inch layers laminated to 1.5 inches, with the outer layer machined as a flush recess so the driver bolts to the inner layer. 8/32 threaded inserts at the driver position, stainless machine screws included.
Windowed U-Brace
A windowed U-brace dadoed to the shell ties the long 41 inch panels together without taking a meaningful bite out of the airspace.
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. Side mounted so it stays accessible with the baffle facing the floor.
Polyfill Installed
Polyfill is installed as a light wall lining to control internal reflections. It does not change the acoustic volume, so do not add more.
Power and Warranty
Hertz rates the MP 300 at 600 watts RMS and 1200 watts peak, and sensitivity is 86 dB at 1W/1m. Most subwoofers in this category sit in the low to mid 80s, so the MP 300 reaches a given output on less amplifier power, which also reduces thermal stress over long sessions.
Both versions are dual voice coil, so final impedance is set at the driver: the D2.3 gives 1 ohm in parallel or 4 ohms in series, the D4.3 gives 2 ohms in parallel or 8 in series. Confirm your amplifier is stable at your chosen load before powering the system.
The enclosure carries a 1 year limited warranty. Bring your own MP 300 D2.3 or D4.3; the driver is purchased separately.
Cut for the Driver, Not the Catalog
Down Fire Wide, 41 inches long and 14 deep, cut for one driver model and built one at a time in Tullahoma, Tennessee. Questions on fitment or amplifier matching, call us at 931-563-5151.
| Series | Proline X Performance Optimized Down Fire Wide |
| Compatible Driver | Hertz Mille Pro MP 300 D2.3 or MP 300 D4.3. Direct fit |
| Enclosure Type | Sealed, down fire, floor coupled |
| Intended Fitment | Where height or depth is the limit and 41 in of length is available |
| Firing Direction | Down firing. Feet ship uninstalled, so front or up firing is also possible |
| External Width | 41 in |
| External Depth | 14 in |
| External Height | 8 in, measured in down fire position, excludes feet |
| Installed Height | 9.75 in with the 1.75 in feet installed |
| Standard 30 x 15 Equivalent | 30 in W x 15 in D x 8.125 in H, 1.173 cubic feet net |
| Internal Height | 5.75 in |
| Internal Volume, Gross | 1.601 cu ft |
| Internal Volume, Net | 1.512 cu ft after the brace and displacement |
| Height Set By | Driver clearance: 0.793 in above the motor, the 0.75 in minimum plus rounding |
| Alignment, MP 300 D4.3 | Qtc 0.694, Fc 39.6 Hz, F3 40.4 Hz, anechoic |
| Alignment, MP 300 D2.3 | Qtc 0.839, Fc 40.2 Hz, F3 34.8 Hz, anechoic |
| Hertz Recommended Sealed Volume | 28 liters. This cabinet is larger, set by motor clearance |
| Driver Fs | 24 Hz (D4.3) / 23 Hz (D2.3) |
| Driver Qts | 0.42 (D4.3) / 0.48 (D2.3) |
| Driver Vas | 74 L (D4.3) / 88 L (D2.3) |
| Driver Effective Diameter | 260mm |
| Driver Xmax | 17mm linear |
| Driver Xmech | 27mm before damage |
| Driver Sensitivity | 86 dB (1W/1m) |
| Driver Power Handling | 600 W RMS / 1200 W peak |
| Driver Voice Coil | 65mm CCAW, 4-layer, vented TIL-P former |
| Driver Cone | V-cone profile, pressed paper pulp with mineral powder injection |
| Driver Surround | High-purity IIR butyl rubber |
| Driver Basket | Six-spoke aluminum alloy with vents under spider |
| Driver Weight | 8.63 kg |
| Voice Coil Configuration | Dual voice coil, impedance set at the driver |
| Mounting Depth | 6.457 in |
| Woofer Opening | 11.3 in |
| Driver Frame Diameter | 12.638 in, sits on the 14 in baffle |
| Driver Displacement | 0.08 cu ft, estimated, no manufacturer figure |
| Material | Langboard Elite 3/4 in MDF |
| Joinery | V-groove and dado, CNC cut on ShopSabre routers |
| 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 the shell |
| Corner Profile | Square, Down Fire series standard |
| Driver Mounting | 8/32 threaded inserts, stainless machine screws included |
| Terminal Cup | One Proline X ABS and carbon fiber composite, side mounted, pre-wired 12 gauge OFC |
| Damping | Polyfill installed as a light wall lining, does not alter acoustic volume |
| Feet | 1.75 in, included, shipped uninstalled |
| Carpet | Plush black |
| Bundle Options | Enclosure only, bring your own Hertz MP 300 |
| Warranty | 1 year limited |
| Built In | Tullahoma, Tennessee |
01Is it a sealed driver?
Efficiency bandwidth product places the motor before anything else is decided.
02What the box does to it
1.512 cu ft against a 74.0 L Vas.
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 17.0 mm — and a bigger one once a subsonic filter stops it wasting travel below the passband.
| Param | Value | Unit | Note |
|---|---|---|---|
| Fs | 24.00 / 23.00 | Hz | free-air resonance |
| Qts | 0.420 / 0.480 | D4.3 / D2.3 | |
| Vas | 74.00 / 88.00 | L | equivalent compliance |
| Sd | 523.7 | cm² | effective piston area |
| Xmax | 17 | mm | Hertz’s D4.3 figure; Hertz publishes no Xmax for the D2.3 |
| Pe | 600 | W | continuous RMS |
| SPL | 85.4 | dB | 1 W / 1 m, derived from T/S |
Driver parameters as published by the Hertz Mille Pro pages at hertz-audio.com. Curves computed against this enclosure’s actual net airspace.
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.
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 |
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.
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.
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.
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.
Handcrafted in USA
Hand-assembled at our facility in Tullahoma, TN. Every enclosure is inspected before it ships.
Tuned to the Build
Internal volume and port geometry are calculated to spec and CNC-cut to exact dimensions. The box performs the same way every time.
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