My personal favorite in this price range is the Yamaha YST-SW315, which you can find online for about $200!
Yamaha YST-SW315
10" Digital Pro Series Subwoofer
MSRP $299.00
-QD-Bass (Quatre Dispersion Bass) Technology
-Advanced Yamaha Active Servo Technology
-A/V ready, magnetically shielded for video use
-Power Built-in 250 Watt high power amplifier
-10" down-firing driver
-Frequency Response: 20Hz-160Hz (+/- 3db)
-Dimensions (W x H X D): 13-3/8" x 17" x 14-9/16"
-Weight: 37.4 lbs.
-Auto power on/off
-Line level and speaker level connections
-40Hz to 140Hz continuously variable high-cut filter
-High cut slope:-24dB/octave
-Volume control
-Phase normal/reverse switch
-BASS selector switch