I have not heard the Axiom, Hsu, or SVS subs. However, I have directly compared Polk, Infinity and Velodyne in the same room and there is no comparison. The Velodyne CHT 12 is so much better than anything Polk or Infinity have in that price range (<$500) that it is not even funny. Others here have compared the Hsu and/or SVS to Velodyne and report that Hsu and SVS are at the head of the class. So, yesterday when a colleague and I were talking about how he might get a new system, I suggested he get Axioms with an SVS sub (based on what I have read around here and elsewhere - plus I own some M22s).

Polk RTi8s - Front
Polk CSi5 - Center
Polk FXi3s - Surrounds
Velodyne CHT 12