If you want a nice tight musical sub that would be good for music as well as HT, consider a used Martin Logan Dynamo which you should be able to find for $400. I would also consider the Emotiva sub www.emotiva.com Mr. P has some good things to say about it.
Mirage makes some nice inexpensive subs as well.

http://emotiva.com/ultra_sub10.shtm

http://www.vanns.com/shop/servlet/it...v_c=GoogleBase

The Mirage may be the better of the 2, but I'm sure both would fit your needs. The Mirage looks to have more power, peak and rms but power isn't every thing. I've heard a Mirage S8 and was impressed with that little sub.

Judging from the size of your room, I would not go with a 12" sub.