Just remember you can always boost the bass settings up in the receiver a couple db to warm up the sound a little. Mp3 at 256 should sound pretty good, I find 128 acceptable as long as the original was a good disc in the first place.

As has been mentioned Hsu, SVS or Paradigm dsp series are very good quality lower cost subs, Axiom's EP350 fits this category very well too.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70