Thank you all very much for your thoughtful and always-amusing replies.
I was only considering the impact of the floor vibrations on the listener, not on other instruments or equipment, so that angle was a revelation to me.
I appreciate you confirming my suspicion that isolating the speakers from the floor might actually be counterproductive to what we're trying to accomplish.
This topic certainly did reinforce for me (again) the notion of how important the room is to sound reproduction, though. It seems that we have to consider not only the shape, size, materials and proportion of the room, but also the elasticity of the floor. Too many variables!