I agree with Barry Kohan's response that decoupling the speakers from the floor is the best way to deal with this problem (mechanical sound transmission). Like Barry I am a professional in this field (I work in Pro A/V system design & installation for the Hollywood studios, networks, etc.). Barry's company Bright Star Audio makes some great products for decoupling speakers and other equipment. A source for sound deadening room treatment materials is Super Soundproofing (www.soundproofing.org). If you can do it, a 1/4" mass-loaded vinyl floor will really reduce the neighbor complaints as it is really good at absorbing the low frequencies (one company calls their mass-load vinyl Dynamat). Other good flooring materials are cork and rubber (again see Super Soundproofing for a excelent cork & rubber sub-floor product). Another great benefit of room treatments is they greatly improve the quality of the sound you hear. Concentrate on the low frequencies - the foam panels that are sold by a number of companies are great for tuning a room for the mid-upper frequencies, but they really do nothing for absorbing the low frequencies and this is what the complaints are about. (FYI -- I am in no way asssociated with Bright Star Audio or Super Soundproofing).
