Short answer: both are discontinued, so this only matters if you are buying used. Used, the Monoprice Monolith 16 THX Ultra is the better pick: it ranks #19 of 143 subwoofers by sound quality (rated 4.4/5), against #88 (rated 4.1/5) for the Monoprice Monolith M15-S.
Buy the Monoprice Monolith 16 THX Ultra only if you find one used in good condition; both are discontinued, and it is the better rated of the two.
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Prices on this page were last checked November 7, 2026 at 7:34 PM UTC. Product prices and availability are accurate as of that date/time and are subject to change; the price and availability shown on Amazon at the time of purchase will apply to the purchase of this product.
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Specifications Side By Side
| Monoprice Monolith 16 THX Ultra | Monoprice Monolith M15-S | |
|---|---|---|
| Price | $2,300 Discontinued | $999 Discontinued |
| Ranked | #19 of 143 in Large Home Subwoofers (Over 75 lbs) | #88 of 143 in Large Home Subwoofers (Over 75 lbs) |
| Enclosure | ported, front | sealed |
| Output SPL at 15 Hz | 107.3 dB | 100.1 dB |
| Output SPL at 20 Hz | 118.4 dB | 105.6 dB |
| Output SPL at 25 Hz | 123.3 dB | 111.2 dB |
| Output SPL at 31.5 Hz | 125.1 dB | 115.2 dB |
| Output SPL at 40 Hz | 126.3 dB | 118.4 dB |
| Output SPL at 50 Hz | 126.5 dB | 119.3 dB |
| Output SPL at 63 Hz | 126.6 dB | 119.6 dB |
| Output SPL at 80 Hz | 126.8 dB | 118.9 dB |
| Output SPL at 100 Hz | 127.2 dB | 119.1 dB |
| Output SPL at 125 Hz | 127.6 dB | 118.9 dB |
| CEA-2010-A Average (40-80Hz) | 126.5 dB | 119.0 dB |
| Dimensions | 25.6”H x 22”W x 28.1”D (650.2 x 558.8 x 713.7 mm) | 23.5”H x 16.7”W x 19.7”D (596.9 x 424.2 x 500.4 mm) |
| Weight | 171.5 lbs (77.8 kg) | 80 lbs (36.3 kg) |
| Warranty | 5 | 5 |
| Cabinet Color | black | - |
| Country | United States | United States |
| Measurement Source | Audioholics | Manufacturer |
| Drivers | 16” (406mm) driver | 15” (381mm) driver |
| Inputs | XLR, RCA, XLR out | - |
| XLR Input | Yes | - |
| RCA Input | Yes | - |
| TRS Input | No | - |
| Optical Input | No | - |
| Coaxial Input | No | - |
| USB Input | No | - |
| HDMI Input | No | - |
| AES Input | No | - |
| Aux Input | No | - |
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