Here are some ideas to research. I have no direct experience with any, so cannot make a recommendation, but these subs seem generally to fit your needs:
Home Theater Direct, Level TWO, $229 + $15 (shipping), www.hometheaterdirect.com
Axiom, EP125, $202 + shipping, www.audioshop.on.ca, 'Nuf said.
Dahlquist, QX-100S, $254 + shipping, www.audioshop.on.ca, Have read good reviews integrating this sub with Axiom bookshelves, ask Marc at Audioshop
AudioSource, SW 10 (down-firing), $150 + $13 shipping, www.myriskfree.com
Energy, S8.2, $239 + $11 shipping, www.audiovideobonanza.com - Goodguys.com has good reviews integrating this sub with Axiom bookshelves
I also have some ideas for subs in the $400 range, if you are interested. Including Aperion Audio, AdireAudio, Athena Technologies
