I picked a PSB Subsonic 7 used for $200. Its a sealed design sporting a 15" woofer and is replacing my PSB Subsonic 5, a ported sub with a 10" driver. Hard to beat for $200.
The SubSonic 7 was the flagship subwoofer of the PSB line. It measures 17.5 inches wide, 19 inches tall and 17.5 inches deep including 1 inch for feet, a basic cube weighing just a little under 60 pounds. The SubSonic 7 has a single 15-inch driver with a rated frequency range of 22Hz to 150Hz, with a low frequency cutoff of 18Hz at –10dB. The SubSonic 7 is driven by an internal MOSFET power amplifier, which is capable of 330 watts continuous and as much as 1200 watts dynamic peak power, able to deliver as much as 112 dB of sound pressure.