Ralph it sort of sounds like you are working with hard floors given your description. What about using a thick layer of putty or elastomer. It seems that one method of this is to couple the vibration to an immovable object (like bookshelves on a stand with blutak) or to provide some isolation from vibration. In this case it seems logical to assume we are trying to decouple the speaker from the floor. One increases coupling and one decouples. How about a experiment where you go over the top in each direction to see which works. One is putty coupling to the floor, while the other is a urethane or something between the speaker and the floor to absorb (decouple) the speaker induced vibrations. Just a thought. Curious to hear what you decide/find. I was not kidding about not hearing anything in my system. That does not mean though that I'm not willing to experiment if someone finds something interesting.
luck
