Jamo 650 Sub for $300 is hard to beat at amazon, as is the Klipsch RW-12D for $300 at newegg. Both on closeout - both originally much higher priced subs.
A couple of us reviewed them here in a small g2g against a DIY combo from Dayton Audio over at avsforum. I can't provide the link because I'm new here, but if you search for Jamo 650 + Klipsch RW-12D at avsforum you'll find the thread.
For the money I don't think you can do better than either of these --- Jamo is flat to 20hz in room, while the Klipsch is flat to 22-24hz in room and has more headroom in the upper frequencies in just default EQ modes. The Klipsch possesses a slightly rising frequency response and is ported, while the Jamo possess a very flat frequency response and is sealed. You'd likely be happy with either (or a pair of either).