Short answer: both are discontinued, and they are rated the same for sound quality (#19 and #25 of 143 subwoofers). Either one is only an option used, so choose on condition and price. If you want one new, the JTR Speakers Orbit Shifter PRO ranks #1 at $4,499.
What the Monoprice Monolith 16 THX Ultra does well: A THX Ultra certified 16 inch ported subwoofer with huge output for far less money than most rivals, now worth finding used. The tradeoff: Heavy at 172 lbs (78 kg) and ported design can interact with room acoustics.
Why pick the JBL 4645C: A pro cinema 18 inch ported subwoofer with very strong upper bass punch, suited to adding impact in a modest room. The catch: Deep-bass output is limited relative to its midbass result, so it is not the right choice for the lowest movie effects.
Buy the Monoprice Monolith 16 THX Ultra or the JBL 4645C, whichever you find used in better condition; both are discontinued. For a new speaker, start with the JTR Speakers Orbit Shifter PRO instead.
Looking for alternatives? See the complete Subwoofers rankings.
A THX Ultra certified 16 inch ported subwoofer with huge output for far less money than most rivals, now worth finding used.
Prices on this page were last checked October 29, 2026 at 4:42 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 | JBL 4645C | |
|---|---|---|
| Price | $2,300 Discontinued | $4,999 Passive, Discontinued |
| Ranked | #19 of 143 in Large Home Subwoofers (Over 75 lbs) | #25 of 143 in Large Home Subwoofers (Over 75 lbs) |
| Enclosure | ported, front | ported, front |
| Output SPL at 15 Hz | 107.3 dB | - |
| Output SPL at 20 Hz | 118.4 dB | 98.9 dB |
| Output SPL at 25 Hz | 123.3 dB | 114.0 dB |
| Output SPL at 31.5 Hz | 125.1 dB | 118.7 dB |
| Output SPL at 40 Hz | 126.3 dB | 122.0 dB |
| Output SPL at 50 Hz | 126.5 dB | 123.7 dB |
| Output SPL at 63 Hz | 126.6 dB | 129.7 dB |
| Output SPL at 80 Hz | 126.8 dB | 133.9 dB |
| Output SPL at 100 Hz | 127.2 dB | 135.1 dB |
| Output SPL at 125 Hz | 127.6 dB | 135.8 dB |
| CEA-2010-A Average (40-80Hz) | 126.5 dB | 127.3 dB |
| Dimensions | 25.6”H x 22”W x 28.1”D (650.2 x 558.8 x 713.7 mm) | 39.7”H x 26.5”W x 17.7”D (1009.6 x 673.1 x 450.8 mm) |
| Weight | 171.5 lbs (77.8 kg) | 138 lbs (62.6 kg) |
| Warranty | 5 | 5/3/2 |
| Cabinet Color | black | black |
| Country | United States | United States |
| Measurement Source | Audioholics | Data-bass |
| Drivers | 16” (406mm) driver | 18” (457mm) 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 | - |
| Passive | - | Yes |
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