I second the big hunk o' rock idea. Even soneting that's 20lbs should help. I would try spikes or cones under a paver slab then compare the speakers on top with spikes and some isolation. I would recomend something other than sorbothane. My experience with wood floors (no carpet) was that bass energy was going directly downward to the floor and then acting like a drum as you describe. So I only worried about verticaly transmited vibrations, and used a platform which I isolated then spiked the speakers into the platform. For isolation I used:
http://www.mapleshaderecords.com/tweaks/isoblocks.html
I bought mine cheap from a supply store that sells parts for heating and cooling contractors. I guess they are commonly used under compressors. They are firm enough that you speaker will not move (clean attack), but they stop most vibration from getting to the floor. With a paver stone the absorb the rest you should be set.
Nik
http://www.mapleshaderecords.com/tweaks/isoblocks.html
I bought mine cheap from a supply store that sells parts for heating and cooling contractors. I guess they are commonly used under compressors. They are firm enough that you speaker will not move (clean attack), but they stop most vibration from getting to the floor. With a paver stone the absorb the rest you should be set.
Nik