Speaker shock absorbers

For a cheap alternative, try your local hardware store. Rubber grommets, o-rings, and various rubber thingies (technical term) in the plumbing department can give you a low-cost way to try various decoupling/resonance damping schemes. I've found all kinds of different thicknesses (or double up with several) and levels of squishiness (another technical term). Good luck, Dave

Well, I can recommend Hockey pucks, a very hard rubber, that I think has worked for me. Cheap! And will maintain speakers inherrent stability.

Now for something a bit more absorbent, are sheets of heavy rubber pads, made I think from recycled rubber, which are primarily intended for garage floors. Very dense and heavy. Trimmed to the size of the bottom of your speaker I doubt that they would alter the stability of the speaker and I'm confident they would be great for isolation in a speaker/floor interface. FWIW, I've started using sheets of this stuff under components which don't use bottom ventilation. So far as I can tell it works very well (I've used sorbothane in the past, and still do).
I got mine from Home Depot - cost about $35 for 6 18" squares (I trim of the interlocking tabs and end up with 14" squares).

FWIW.

As to weight of the speakers - they are 90 lbs each. The vibration is not bad - the speaker cabinet does not noticably vibrate. But, at 105 db peaks with my feet on the carpeted wooden floor, sitting 6 feet away, I can feel the base. It seems to me that completely eliminating any transmission along the floor would be ideal. I cannot imagine that this would not occur in even the heaviest of speakers without decoupling. The garage rubber pads sound like just the ticket - I'll check out home depot - the hocky pucks are also an interesting idea.

Musicnoise, some of what you are feeling is the result of airborne bass soundwaves, not necessarily as a result of the speaker/floor interface. And airborne soundwaves, especially bass ones are tough to deal with unless you have sound absorbers of some sort in the room, the the sound absorbers for bass would have to be huge. FWIW.

Musicnoise - hockeypucks might be to hard. Vibrapods are constructed of special rubber specially for that purpose. My speakers are also around 90 lbs and very rigid - (a lot of cross boards inside) but floor is not (basement). Originally I had spikes. Idea of the spike is that infinitely small point cannot transfer energy. In practice it does but shifts resonance frequency up. Regular rubber bounces and felt compresses. Rubber and construction of vibrapods create big losses (no energy transfer). Similar rubber is used in pro computers to isolate drive cage from the rest of the computer. The best is to use softer vibrapods and more of them but it costs.