Sorbathane under granite slabs for speaker bases?

My floors are raised wood. I  had a hell of a time getting the speakers to sound right.

 I tried just spiking them through the carpet and into the wood floors. Undefined  bass. 

Then I tried them spiked to granite bases.  Improved mids and highs. No bass. 

Finally, I bought a pair of Maple Shade unfinished maple platforms. I spiked the platforms to the floor through the carpet, then spiked the speakers to the maple platforms. Walla! Done. 

Double click on my icon, then double click again to see the Maple platforms under the speakers. 

Frank

agree with Mapleshade, avoid granite

The very expensive TMC isolation platform uses a thick granite top plate. I have used thick granite and bluestone with springs, sonically exceptional in every way. High stiffness and high mass on springs, cannot beat with stick. Granite cannot ring when it's isolated. Hel-loo!

how much is the TMC ?

If I read your proposed installation correctly, the unusual variable I doubt any of us has worked with is the "hollow five inch high tile shelf" that your speakers will be placed on. Your speakers are fairly heavy (75 lbs. or so) and it has been my experience that heavier speakers can do fine on granite, lighter weight speakers perform better coupled to wood platforms. That's just my experience, there are always outliers and ears that hear things differently. I would try both the spikes and the glider feet that your speakers came with, the spikes might not necessarily sound better. You might find that using Herbies products under both the granite plinths and the speakers would prove optimal with that hollow tile shelf. I've never tried sorbothane products for isolation, mainly because I've never known anyone to have anything positive to say about the material for this type of application.