Short answer: both are discontinued, so this only matters if you are buying used. Used, the Outlaw Audio Ultra-X12 is the better pick: it ranks #7 of 143 subwoofers by sound quality (rated 4.2/5), against #88 (rated 4.1/5) for the Monoprice Monolith M15-S. If you want one new, the HSU Research ULS-15 MK2 ranks #4 at $999.
Buy the Outlaw Audio Ultra-X12 only if you find one used in good condition; both are discontinued, and it is the better rated of the two. For a new speaker, start with the HSU Research ULS-15 MK2 instead.
Looking for alternatives? See the complete Subwoofers rankings.
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Specifications Side By Side
| Monoprice Monolith M15-S | Outlaw Audio Ultra-X12 | |
|---|---|---|
| Price | $999 Discontinued | $999 Discontinued |
| Ranked | #88 of 143 in Large Home Subwoofers (Over 75 lbs) | #7 of 212 in Medium Home Subwoofers (30-75 lbs) |
| Enclosure | sealed | ported, bottom |
| Output SPL at 15 Hz | 100.1 dB | - |
| Output SPL at 20 Hz | 105.6 dB | 108.3 dB |
| Output SPL at 25 Hz | 111.2 dB | 115.0 dB |
| Output SPL at 31.5 Hz | 115.2 dB | 116.1 dB |
| Output SPL at 40 Hz | 118.4 dB | 117.8 dB |
| Output SPL at 50 Hz | 119.3 dB | 121.4 dB |
| Output SPL at 63 Hz | 119.6 dB | 122.3 dB |
| Output SPL at 80 Hz | 118.9 dB | - |
| Output SPL at 100 Hz | 119.1 dB | - |
| Output SPL at 125 Hz | 118.9 dB | - |
| CEA-2010-A Average (40-80Hz) | 119.0 dB | 120.5 dB (avg of 3/4 bands) |
| Dimensions | 23.5”H x 16.7”W x 19.7”D (596.9 x 424.2 x 500.4 mm) | 21.3”H x 16”W x 23”D (539.8 x 406.4 x 584.2 mm) |
| Warranty | 5 | 3 |
| Country | United States | United States |
| Measurement Source | Manufacturer | Brent Butterworth |
| Weight | 80 lbs (36.3 kg) | 65.9 lbs (29.9 kg) |
| Drivers | 15” (381mm) driver | 12” (305mm) driver |
| Cabinet Color | - | black |
| Frequency Response | - | 19-179Hz, ±3dB |
| Amplifier Power | - | 350W RMS |
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