Proline X
Performance Optimized Down Fire Wide Sealed 12" Enclosure | Hertz Mille Pro MPS 300
SKU: PO 12-S DFW MPS
- Alignment: 0.800 cu ft net. MPS 300 S2 Qtc 1.002, Fc 47.2 Hz, F3 37.0 Hz; MPS 300 S4 Qtc 0.812, Fc 44.5 Hz, F3 39.5 Hz
- Down Fire Wide: 41"W x 14"D x 5.4375"H in down fire position, lower than the standard 30 x 15 (6.375 in)
- Height Set By: the airspace, matched to the standard 30 x 15 box
- Driver: Hertz MPS 300 S2 (2 ohm) or S4 (4 ohm) SVC, 500W RMS / 1000W peak, 16.8mm Xmax, 3.465" mounting depth
- Build: V-groove and dado joinery, 1.5" double baffle, 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 MPS 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 MPS 300, S2 or S4
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 MPS 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 MPS 300 frame measures 12.532 inches across and sits on the 14 inch deep baffle with about 0.73 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 5.4375 inches tall, against 6.375 for the standard 30 x 15 Hertz Mille Pro MPS 300 cabinet, both measured in the down fire position without feet. That is 0.9375 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 7.1875 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 MPS 300's own published numbers. A Performance box fits your subwoofer. A Performance Optimized box was built for it.
Built Around the MPS 300
The baffle is bored to the Hertz 283mm cutout, 11.14 inches, and the internal height clears the driver's 3.465 inch mounting depth with 1.223 inches to spare. The result is a full 12 in a cabinet that stands 5.4375 inches tall.
The MPS 300 carries 16.8mm of Xmax, an unusually high figure for a shallow driver, on a polypropylene cone with a 38mm voice coil and a high density flux ferrite motor. It is rated 500 watts RMS and 1000 watts peak. Hertz includes a mesh grille in the box that can be rotated 90 degrees or removed.
What makes this driver reach is the suspension: a 24 Hz free-air resonance on the S2 with 64.9 liters of Vas, and 23 Hz with 62.1 liters on the S4. Both models are single voice coil, so impedance is fixed by the model: 2 ohms on the MPS 300 S2, 4 ohms on the MPS 300 S4. Hertz states Fs, Vas and Qts are unchanged between parallel and series measurement, so one cabinet is correct either way.
Net Volume and Alignment
The cabinet holds 0.800 cubic feet net, from 0.888 cubic feet gross after the brace and the driver's 0.08 cubic feet of displacement (estimated, no manufacturer figure).
Height is the free dimension. With the footprint fixed at 41 by 14, the cabinet height was solved to put the cabinet on the same airspace as the standard 30 x 15 box, which holds 0.80 cubic feet net, and the motor clears the top panel with 1.223 inches to spare. Same air, same alignment, lower cabinet.
Computed from Hertz's published Thiele-Small parameters: MPS 300 S2: Qtc 1.002, Fc 47.2 Hz, F3 37.0 Hz; MPS 300 S4: Qtc 0.812, Fc 44.5 Hz, F3 39.5 Hz.
That matches Hertz's own sealed recommendation of 22.7 liters. Pay attention to the model choice, because it reverses from the 10 inch MPS 250: on the MPS 300 the S4 is the better damped driver, Qts 0.42 against 0.51, with far more motor force, 18.7 T·m against 12.3. The S4 is the controlled alignment; the S2 reaches about 2.5 Hz deeper with a modest lift at the corner.
Hertz's published enclosure data reconciles with its own T/S parameters: run their Qts and Vas through standard sealed box math at their recommended volume and the corner frequency comes back where they print it. That is not true of every manufacturer in this category.
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 5.4375 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 7.1875 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.
Down Firing Into the Floor
The driver fires downward into the floor and the cabinet couples to the surface it sits on. The sealed rolloff meets rising cabin gain below 60 Hz, so in-car response extends below the anechoic figures quoted above.
For sealed applications Hertz suggests a 15 Hz subsonic filter at 24 dB per octave and a 70 to 80 Hz low pass, also at 24 dB per octave. Those are the manufacturer's own recommended settings for this driver.
Power and Warranty
The MPS 300 is rated 500 watts RMS and 1000 watts peak on a 38mm voice coil. Since both models are single voice coil, the load your amplifier sees is decided when you choose the S2 or the S4. Check what your amplifier is stable into before you order the driver.
The enclosure carries a 1 year limited warranty. Bring your own MPS 300 S2 or MPS 300 S4; 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 MPS 300 S2 (2 ohm SVC) or MPS 300 S4 (4 ohm SVC). 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 | 5.4375 in, measured in down fire position, excludes feet |
| Installed Height | 7.1875 in with the 1.75 in feet installed |
| Standard 30 x 15 Equivalent | 30 in W x 15 in D x 6.375 in H, 0.80 cu ft net |
| Internal Height | 3.1875 in |
| Internal Volume, Gross | 0.888 cu ft |
| Internal Volume, Net | 0.800 cu ft after the brace and displacement |
| Height Set By | Airspace; the motor clears the top panel with 1.223 in |
| Alignment, MPS 300 S2 | Qtc 1.002, Fc 47.2 Hz, F3 37.0 Hz, anechoic |
| Alignment, MPS 300 S4 | Qtc 0.812, Fc 44.5 Hz, F3 39.5 Hz, anechoic |
| Hertz Recommended Sealed Volume | 22.7 liters per driver |
| Driver Fs | 24 Hz (S2) / 23 Hz (S4) |
| Driver Qts | 0.51 (S2) / 0.42 (S4), S4 is the better damped on this size |
| Driver Qes / Qms | 0.55 / 7.5 (S2) | 0.45 / 6.3 (S4) |
| Driver Vas | 64.9 L (S2) / 62.1 L (S4) |
| Driver BL | 12.3 T·m (S2) / 18.7 T·m (S4) |
| Driver Mms | 295 g (S2) / 320 g (S4) |
| Driver Xmax | 16.8mm |
| Driver Sensitivity | 84.5 dB (S2) / 85 dB (S4) |
| Driver Voice Coil | 38mm, single voice coil |
| Driver Motor | High density flux ferrite |
| Suggested Filters | 15 Hz subsonic at 24 dB/oct, 70 to 80 Hz low pass at 24 dB/oct (Hertz) |
| Driver Cms | 0.16 mm/N (S2) / 0.15 mm/N (S4) |
| Driver Power Handling | 500 W RMS / 1000 W peak |
| Voice Coil Configuration | Single voice coil, impedance set by driver model |
| Mounting Depth | 3.465 in |
| Woofer Opening | 11.14 in |
| Driver Frame Diameter | 12.532 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 MPS 300 S2 or S4 |
| 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
0.800 cu ft against a 64.9 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 16.8 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 | 0.510 / 0.420 | S2 / S4 | |
| Vas | 64.90 / 62.10 | L | equivalent compliance |
| Sd | 541.6 | cm² | effective piston area |
| Xmax | 16.8 | mm | one-way linear travel |
| Pe | 500 | W | continuous RMS |
| SPL | 84.0 | dB | 1 W / 1 m, derived from T/S |
Driver parameters as published by Hertz Mille PRO technical sheet. 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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