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 - 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 - tallest in the Down Fire line
Financing available — choose Shop Pay Installments at checkout.
Performance Optimized Down Fire Sealed Dual 12" Enclosure | Hertz Mille Pro MPS 300
The Performance Optimized Down Fire Sealed Dual 12" Enclosure for the Hertz Mille Pro MPS 300 is a two-chamber sealed cabinet built around a pair of Hertz shallow 12s. At 10.75 inches tall on a 30 by 15 inch footprint, it delivers roughly 6 dB more maximum output than a single driver in the same alignment.
Each Hertz MPS 300 is available as the MPS 300 S2 (2Ω) or MPS 300 S4 (4Ω), rated 500 watts RMS and 1000 watts peak with a 3.46 inch mounting depth. Across the pair that is 1000 watts RMS combined. Each driver carries 16.8mm of Xmax, with a polypropylene cone, 38mm voice coil and high density flux ferrite motor.
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. Two drivers sharing one sealed volume interact acoustically, with each driver's compression phase becoming the other's pressure environment, which smears transient response and blurs the alignment each would have on its own. Separate chambers eliminate that entirely.
Each chamber gives 0.80 cubic feet net per driver, matching Hertz's own sealed recommendation of 22.7 liters. That produces 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, identical to the single-driver cabinet.
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 Qts of 0.42 against the S2's 0.51. On the MPS 250 it is the opposite. The S4 gives the more controlled alignment, the S2 reaches about 2.4 Hz lower. Whichever you choose, use two of the same model.
At 37.1 Hz on the S2 this is among the deepest-reaching enclosures in the Proline X Down Fire line, and it comes from a 24 Hz free-air resonance paired with a 64.9 liter Vas.
Hertz publishes Thiele-Small measured with coils in parallel and states Fs, Vas and Qts are identical in series wiring. One cabinet is correct for either configuration. Their published enclosure data also reconciles with their own T/S parameters, which is not true of every manufacturer in this category.
This is the tallest cabinet in the Down Fire line at 10.75 inches. It is a cargo floor or rear-wall build rather than an under-seat one. Measure your vertical clearance before ordering.
Construction is the full Proline X Performance Optimized platform: ShopSabre CNC-cut Langboard Elite MDF with V-groove and dado joinery, perimeter cross-bracing, and double-layer 1.5 inch front baffles (two 3/4 inch sheets bonded) at both driver positions. Corners are square on Down Fire cabinets. Both drivers fire down into the floor, and one side-mounted Proline X ABS/carbon fiber composite terminal cup serves both chambers, fed in parallel to both driver positions with 12 gauge OFC.
Bring your own pair of MPS 300 S2 or MPS 300 S4. Both are single voice coil, so your final load is determined by which model you install and how the pair is wired at the amplifier.
Manufactured in Tullahoma, Tennessee. 1-year enclosure warranty.
| Enclosure Type | Sealed | Dual Driver | Down Fire Geometry | Floor-Coupled |
| Chamber Configuration | Two Fully Separate Chambers | Full Divider | Single Shared Side-Mounted Terminal Cup |
| Driver-Specific Fitment | Hertz MPS 300 S2 (2Ω SVC) or MPS 300 S4 (4Ω SVC) | Pair Required |
| 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 |
| Target Qtc Per Chamber (S2) | 1.003 |
| Target Qtc Per Chamber (S4) | 0.812 |
| 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 |
| Power Handling | 500 Watts RMS Each / 1000 Watts RMS Combined |
| Mounting Depth Required | 3.46 Inches (Hertz Spec) |
| Cutout Diameter (Each) | 283mm / 11.14 Inches (Hertz Spec) |
| Suggested Filters | 15 Hz Subsonic at 24 dB/Oct | 70-80 Hz Low Pass at 24 dB/Oct (Hertz) |
| Baffle Configuration | Down Firing - Both Drivers Fire Toward Floor |
| Material | Langboard Elite 3/4" MDF |
| Front Baffle Construction | Double-Layer 1.5" MDF at Both Driver Positions |
| Chamber Divider | 3/4" MDF Full Bonded Panel |
| Joinery | V-Groove and Dado (CNC-Cut) |
| Corner Profile | Square (Down Fire Series Standard) |
| Construction | ShopSabre CNC / Perimeter Cross-Brace |
| External Width | 30 Inches |
| External Depth | 15 Inches |
| External Height | 10.75 Inches |
| Dimension Reference | All External Dimensions Measured In Down Fire Position |
| Voice Coil Configuration | Single Voice Coil Per Driver - Impedance Set By Driver Model |
| Mounting Hardware | 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 |
| Damping | Polyfill Pre-Installed Per Chamber |
| Carpet Finish | Dark Gray Trunkliner (Standard) or Plush Black (+$10) |
| Bundle Options | Enclosure Only (Bring Your Own Pair of MPS 300 S2 or S4) |
| Warranty | 1-Year Limited (Enclosure) |
| Manufactured | Tullahoma, Tennessee, USA |
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
