On the one hand, there is the PB-1000. This sub features a bass reflex design, which combines a 10-inch front facing driver, with a 3.5-inch port for extended bass response. The PB-1000, via its built-in Sledge STA-300D amplifier, can deliver an continuous output of 300 watts, and as much 720 watts peak power when needed. The frequency response for the PB-1000 is listed as 19Hz to 270Hz (+ or - 3db).
The other subwoofer choice you have for $499 is the SB-1000. This unit features a larger 12-inch front facing driver in a sealed cabinet (no port), but is also supported by the same amplifier that is included with the PB-1000. However, the amp is configured differently to take into account the difference in driver size and cabinet design. Frequency response for the SB-1000 is listed as 24Hz to 260Hz (+ or - 3db).
One thing to point out is that even though the PB-1000 features a smaller driver, the SB-1000 actually has a smaller cabinet that measures only 13-inches square, and also weighs in less (at 27lbs) than the PB-1000 (which weighs 46lbs). On the other hand, the PB-1000, with its combination of a 10-inch driver and supplemental port, is actually rated to deliver slightly lower frequencies.
Update 2/8/13: SVS SB-1000 12-inch Compact Subwoofer Reviewed.