Definitely give a listen to the Athena subs. I have the AS-P400, which I'm using with M22's in a music-only set up. If I remember correctly this sub listed around $400 but I found a couple of brick-and-mortar stores that sold it for $320 - neither was a Best Buy. I remain very enthusiastic about this sub. Very fast, very musical. Not sure of your taste in music but we listen to two types of music - classical and (very loud) alternative rock. It's very good with either - no staining or churffing with the rock and articulate enough to reproduce the bowing of a big orchestral bass and not just a rumble at the right frequency. The following is a link to a professional review:
http://www.hometheatersound.com/equipment/athena_audition_asp400.htm

Another sub you might listen to is the Def Tech ProSub100. Excellent sound and a bit more money; I think I had quotes in the $360-390 range for this one.

Happy listening and let us know of any other gems you come across in your search.