This is the problem I had with my M22s I had a $500 or less budget and Axiom sub cost too much.
Cheap subs just didn't cut it. The Velodyne VX-10 is OK but doesn't go too low, and couldn't keep up with the M22s at higher volumes. I wound up with a Paradigm DSP-3100 and am very happy. $450 from my local dealer. I don't feel right posting this at the axiom forum, but I do think Axiom should make a sub in that price range. If only the EP175 would come down in price a bit... I would have gone for axiom for sure.