Proline X
Performance Optimized Down Fire Sealed Dual 12" Enclosure | Hertz Mille Pro MPS 300
SKU: PO 12D-S DF MPS
- Alignment: 0.80 cu ft net per chamber, 1.60 cu ft net total, Hertz's own spec. Qtc 1.003 / F3 37.1 Hz (S2), Qtc 0.812 / F3 39.5 Hz (S4)
- Chambers: Two fully separate sealed chambers, 3/4" MDF full divider, one shared side-mounted terminal cup
- Drivers: Pair of Hertz MPS 300 S2 or S4 (SVC), 500W RMS each / 1000W combined, 16.8mm Xmax
- Cabinet: 30"W x 15"D x 10.75"H measured in down fire position, square corners
Financing available. Choose Shop Pay Installments at checkout.
Performance Optimized Down Fire Sealed Dual 12" Enclosure | Hertz Mille Pro MPS 300
Proline X Performance Optimized Down Fire Dual 12" Sealed
Cut and tuned for a pair of Hertz Mille Pro MPS 300
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, with cabinet height as the variable. A few drivers need a wider or deeper platform. | Compact per driver footprint. 21.5 by 14.5 inches on the 10 inch. |
| 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. | 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 airspace, the cutout and the cabinet height 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 a Pair of MPS 300s
Both baffles are bored to the MPS 300's 283mm cutout, 11.14 inches, and the internal height is set by the driver's 3.46 inch mounting depth. The cabinet stands 10.75 inches tall on the standard 30 by 15 inch footprint, and delivers roughly 6 dB more maximum output than a single driver in the same alignment.
Each MPS 300 carries 16.8mm of one way Xmax behind a 38mm voice coil and a high density flux ferrite motor, with a polypropylene cone. Power handling is 500 watts RMS and 1000 watts peak per driver, 1000 watts RMS across the pair.
At 37.1 Hz per driver on the S2, this is among the deepest reaching enclosures in the Proline X Down Fire line, and here you get it twice. That depth comes from a 24 Hz free air resonance paired with a 64.9 liter Vas in a driver that mounts in 3.46 inches. Each driver is sold as the MPS 300 S2 at 2 ohms or the MPS 300 S4 at 4 ohms, both single voice coil. Use two of the same model; mismatched drivers between the two chambers will not track each other.
Net Volume, Not Gross
Each chamber is built at 0.80 cubic feet net per driver, 1.60 cubic feet net total, out of a 1.78 cubic foot gross cavity. That per driver figure matches Hertz's own sealed recommendation of 22.7 liters, so each chamber lands on the same alignment as the single driver cabinet: Qtc 1.003 with F3 at 37.1 Hz on the S2, or Qtc 0.812 with F3 at 39.5 Hz on the S4.
True net, not marketing net. Internal volume is calculated from the machined cavity dimensions and the driver's published displacement, not estimated from external dimensions minus wall thickness.
Pay attention to the coil choice, because it reverses from the 10 inch model. On the MPS 300 the S4 is the better damped driver, with a free air Qts of 0.42 against the S2's 0.51, and it carries far more motor force, 18.7 T·m against 12.3. The S4 gives the more controlled alignment; the S2 reaches roughly 2.4 Hz lower. On the MPS 250 the pattern is the opposite. Same series, reversed between sizes.
Two Chambers, Not One Volume
This enclosure follows the Proline X standing design rule for dual sealed builds: two fully separate chambers divided by a bonded full height 3/4 inch MDF panel, giving each driver its own dedicated airspace and its own polyfill.
Two drivers sharing one sealed volume interact acoustically. Each driver's compression phase becomes the pressure environment the other is working against, which smears transient response and blurs the alignment each would have on its own. A full divider eliminates that entirely, and each MPS 300 sees exactly the 0.80 cubic feet Hertz specifies.
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 both chambers airtight under sustained pressure cycling. Corners are square on Down Fire cabinets.
Be realistic about size. At 10.75 inches this is the tallest cabinet in the Down Fire program. It is a cargo floor, rear wall or behind the bench build rather than an under seat one. The 30 by 15 inch footprint is unchanged, so it takes no more floor area than any other Down Fire cabinet, it simply stands taller to give each driver its full 0.80 cubic feet. Measure your vertical clearance before ordering.
Double Baffle, Flush Recess
Both front baffles are 1.5 inches, two bonded 3/4 inch sheets, with the outer layer machined as a flush recess so each driver bolts to the inner layer. That recovers 3/4 inch of internal clearance at each position, and it matters with two 11 inch cutouts in one cabinet.
Side-Mounted Terminal Cup
One Proline X ABS and carbon fiber composite cup with stainless hardware and copper ring terminals serves both chambers, pre-wired in 12 gauge OFC to both driver positions in parallel. Side mounted so it stays accessible with the baffles facing the floor.
Separate Chambers
A full height bonded 3/4 inch MDF divider splits the cabinet into two sealed chambers of 0.80 cubic feet net each. Perimeter cross-bracing, polyfill in each chamber, and two sets of 8/32 threaded inserts with stainless machine screws.
Power and Warranty
Each driver is rated 500 watts RMS and 1000 watts peak, 1000 watts RMS combined. Both models are single voice coil, so there is no coil wiring decision at the driver; your final load is set by which model you install and how the pair resolves at the amplifier. Confirm that against your amplifier's minimum stable load before powering up.
Hertz publishes Thiele-Small measured with coils in parallel and states that Fs, Vas and Qts are identical in series wiring, and their published enclosure data reconciles with their own T/S parameters, which is not true of every manufacturer in this category. Firing down into the floor, the sealed rolloff meets rising cabin gain below 60 Hz. Hertz suggests a 15 Hz subsonic filter at 24 dB per octave and a 70 to 80 Hz low pass for sealed applications.
The enclosure carries a 1 year limited warranty. Bring your own pair of MPS 300 S2 or MPS 300 S4; the drivers are purchased separately. Assembled and inspected in Tullahoma, Tennessee.
Cut for the Driver, Not the Catalog
One cutout, one mounting depth, one published airspace per chamber. Every Performance Optimized cabinet is machined to a single driver's numbers and built one at a time in Tullahoma, Tennessee.
| Series | Proline X Performance Optimized Down Fire |
| Compatible Driver | Hertz MPS 300 S2 (2Ω SVC) or MPS 300 S4 (4Ω SVC) | Pair Required |
| Enclosure Type | Sealed | Dual Driver | Down Fire Geometry | Floor-Coupled |
| Chamber Configuration | Two Fully Separate Chambers | Full Divider | Single Shared Side-Mounted Terminal Cup |
| Firing Direction | Down Firing, Both Drivers Fire Toward Floor |
| External Width | 30 Inches |
| External Depth | 15 Inches |
| External Height | 10.75 Inches |
| Dimension Reference | All External Dimensions Measured In Down Fire Position |
| Total Internal Gross Volume | 1.78 Cubic Feet |
| Per-Chamber Net Volume | 0.80 Cubic Feet, Matches Hertz Spec of 22.7 Liters |
| Total Net Volume | 1.60 Cubic Feet |
| Manufacturer Target Volume | 22.7 Liters Per Driver (Hertz Sealed Recommendation) |
| Target Qtc Per Chamber (S2) | 1.003 |
| Target Qtc Per Chamber (S4) | 0.812 |
| Predicted Fc Per Chamber (S4) | 44 Hz |
| Predicted F3 Per Driver (S2) | 37.1 Hz |
| Predicted F3 Per Driver (S4) | 39.5 Hz |
| 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 Vas | 64.9 Liters (S2) / 62.1 Liters (S4) |
| Driver BL | 12.3 T·m (S2) / 18.7 T·m (S4) |
| Driver Xmax | 16.8mm |
| Driver Voice Coil | 38mm | Single Voice Coil Per Driver |
| Driver Motor | High Density Flux Ferrite |
| Power Handling | 500 Watts RMS Each / 1000 Watts RMS Combined |
| Mounting Depth Required | 3.46 Inches (Hertz Spec) |
| Woofer Opening (Each) | 283mm / 11.14 Inches Cutout Diameter (Hertz Spec) |
| Standoffs | None, Cabinet Sits Flat |
| Material | Langboard Elite 3/4" MDF |
| Construction | ShopSabre CNC / Perimeter Cross-Brace |
| Joinery | V-Groove and Dado (CNC-Cut) |
| Panel Thickness | 3/4 Inch |
| Front Baffle | Double-Layer 1.5" MDF at Both Driver Positions |
| Chamber Divider | 3/4" MDF Full Bonded Panel |
| Corner Profile | Square (Down Fire Series Standard) |
| Driver Mounting | 8/32 Threaded Inserts | Stainless Steel Machine Screws Included | 2 Sets |
| Terminal Cup | One Proline X ABS/Carbon Fiber Composite | Side Mounted | 12g OFC Pre-Wired to Both Driver Positions |
| Internal Wiring | 12 Gauge OFC, Parallel to Both Driver Positions |
| Damping | Polyfill Pre-Installed Per Chamber |
| Carpet Finish | Plush Black |
| Suggested Filters | 15 Hz Subsonic at 24 dB/Oct | 70-80 Hz Low Pass at 24 dB/Oct (Hertz) |
| Voice Coil Configuration | Single Voice Coil Per Driver, Impedance Set By Driver Model |
| Bundle Options | Enclosure Only (Bring Your Own Pair of MPS 300 S2 or S4) |
| Weight | 46 lb |
| Warranty | 1-Year Limited (Enclosure) |
| Built In | Tullahoma, Tennessee, USA |
01Is it a sealed driver?
Efficiency bandwidth product places the motor before anything else is decided.
02What the box does to it
0.80 cu ft against a 62.1 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 | 23.00 | Hz | free-air resonance |
| Qts | 0.420 | SVC 4 | |
| Vas | 62.13 | L | equivalent compliance |
| Sd | 541.6 | cm² | effective piston area |
| Xmax | 16.8 | mm | one-way linear travel |
| Pe | 500 | W | continuous RMS |
| SPL | 84.1 | 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.
Shop By Series
