I live in an apartment with concrete subfloors, and have had problems with the bass from my Hsu STF-2 disturbing the lateral neighbors. (Concrete actually is a pretty good conductor.)
One thig that helped was that I installed a GRAMMA riser from Auralex under the subwoofer.
http://www.auralex.com/sound_isolation_gramma/sound_isolation_gramma.asp At $50, it's a little overpriced, but it works.
Another nice benefit of the GRAMMA, which isolates the subwoofer from the floor, is that the sound reproduction is much cleaner and tighter.
