Sub from a different manufacturer
I completely agree with ThisGuy. I auditioned many of the most popular and highly rated speakers on the web - Klipsch, Axiom, PSB, Energy, Polk, etc. - but none of these manufacturers could make a sub like Hsu, SVS, Earthquake, Velodyne, etc. If you really want the best bass, you really should audition other brands from your speaker manufacturer.
Just out of curiosity, what kinds of speakers do you have and what subs are you considering?