advice on replacing a sub woofer

I have the Athena Technology Micra M225 Surround Sound system. The subwoofer on it has broken. Can anyone recommend a substitute so that I do not have to replace the entire system. (Looking to spend no more than $300; used is fine). This is strictly used for home theater in a medium sized living room. Here are the specs:

Surround System:
http://www.athenaspeakers.com/products/m225-specifications/

Amp:Yamaha RX-V663:
� Minimum RMS Output Power for Front, Center, Surround, Surround back
20 Hz to 20 kHz, 0.06% THD, 8 Ω ...................................... 95 W � Dynamic Power (IHF)
Front L/R, 8/6/4/2 Ω ..................................... 135/165/195/240 W
� Maximum Useful Output Power (JEITA) [Asia, General, China and Korea models]
1 kHz, 10% THD, 8 Ω .......................................................................................... 130 W

� Rated Output Voltage/Output Impedance OUT (REC) .......................................................... 200 mV/1.2 kΩ PRE OUT ................................................................. 1.0 V/1.2 kΩ SUBWOOFER (2ch Stereo, FRONT SP: SMALL)

Thank you