Sorbathane under granite slabs for speaker bases?

Well, transforming vibration to heat is what ANY constrained layer viscoelastic type damper does, but Sorbothane is not as good at this as many other materials for whatever set of reasons. Any major dude will tell you. Certainly Sorbothane doesn’t isolate down to 5 Hz. That’s hooey. In the overall scale of things I’m inclined to say Sorbothane is one of those materials a lot like lead that appears to be a good idea but in practice actually hurts the sound. Great for running shoes, though.

Sorbothane, or any elastic substance will allow the speaker to move - esp. when a bass note at high volume is sounded.  This is a no-no in any box speaker, as that will phase-modulate all of the sound.

If you have box speakers, you want spikes.

" Any major dude will tell you."  I prefer my advice from non-dudes!

I use spikes on my main speakers.  However I have to say that the sorbothane applied to the front of the speakers, especially the 1/2" 70 duro, does far more for the sound than the spikes. As regard putting sorb footers under speakers, people have been doing it for a long time and many report good results.  But I tend to agree with the above comment about allowing the speaker to move. Of course many things that would seem to be a problem turn out not to be so.  Given the positive reports, I wouldn't be dogmatic about not trying sorb footers.