Sorbathane under granite slabs for speaker bases?

The same reason Sorbothane is used for shoe insoles - shock absorption - makes it unsuitable for use in isolation devices since Sorbothane stores energy, preventing energy from rapidly exiting the system. Compared to what, you ask? Compared to say, Super Balls. Super Balls release energy very rapidly. Or very hard cones, which also release energy rapidly. It’s a subtle thing, the Sorbothane, but quite noticeable compared to real isolation. Tone incorrect, loss of dynamics, closed in sound. Ah, I remember it well.

Dill, so I should use something under the granite?

I guess the question is whether to drain or isolate.  Personally, I've avoided visco-elastic stuff under speakers.   Expect others have opposite experience to share.

I'm using hardwood (3"?) plinths directly under speakers.  Hardwood resting on large 18x18" tile.  Tile on carpet.  The tile gives the plinth stability on the carpet.  The hardwood "drains" vibrational energy from the speakers (or so I think). 

Thanks ghost,

Are you using spikes as well? I think I'm going to roll with just the granite slabs directly on the tile shelf w/spikes under the KEFs.

Always good to experiment, ahahitoro.  Plain granite now doesn't eliminate the possibility of later trying some wood in between.  

To answer your question...yes, I use the spikes as well.  With the Silverline Preludes, I use the spikes they supply though have installed those little dimpled discs to keep the spike from digging into the wood.  With the Forests, they have a "ball & claw" arrangement for coupling.  The balls rest directly on the wood.