Sorbathane under granite slabs for speaker bases?

Try Townshend Audio Seismic Podium......these are customized to speaker weight and I believe are right up there in terms of speaker isolation solutions....I haven't tried them myself but those who have swear by it. I have a pair on order though.......

cheers

hi 
if you want to isolate your speakers down to 3hz ,then i second Townsend audio seismic podiums , since Emporium hifi in the uk installed  a pair under my soundlabs my system has improved to levels i didn't know was possible ,
by stopping the seismic vibrations from your speakers travelling threw the floor and up into your equipment , i believe without this vibration your loudspeakers are free to breath and your electronics sound much better as well , it has a very positive knock on effect on the rest of your system
i have never experienced an upgrade in sound quality like it ,
go and audition a pair , they are sold in the uk with up to 40% discount on all sizes to order you need the weight of each speaker and the foot print size,
i tried various cones, squidgy stuff and bearings nothing comes close , some of the bearing ideas where more than twice the price
check Max Townsend you tube video showing the effects of the positive effects of seismic podiums compared to the damage spikes cause , he has proved using spikes under loudspeakers is the worse thing you can do for your loudspeakers and hifi system ,
check out the video you will never use spikes again and will end up buying podiums like so many other audiophiles around the world 
they are a huge break threw in high end audio they are just as important as the speakers them selves

for a cheaper option you can use your granite , but use the seismic cells underneath , you will just need to work out the weight of the loudspeaker adding on the weight of the granite , then you have basically made your own version of the podiums at a bargain price 
the seismic load cells (pods) can be made to isolate any weight , i would recommend a set of 4 under each granite platform , do not use your spikes ,just place your speakers straight on top of the granite or you can use squidgy pads to dampen any ringing between the granite and the base of your speakers good luck 

When granite is used with springs you get the benefit of high mass and low resonant frequency - below 3 Hz if you play your cards right. As a bonus there are no resonance issues with granite since it's isolated along with the component, thus no reason to apply damping, always a plus in these delicate matters. The inexpensive way to play with the big boys. Certainly no reason to spend a fortune. The only issue for springs is center of gravity, I.e., tall heavy speakers might be too top-heavy to place springs directly under them. But there is more than one way to skin a cat. Subwoofers are ideal since they have low COG.

geoff kait
machina dynamica
isolation & resonance control

The Townshend Seismic isloators are also available separately, as individual "Pods". Using three or four of them under a speaker (or any other component) provides isolation to a very low frequency, and is cheaper than the Seismic Shelves themselves, which are not really required---the Pods are doing all the isolating, not the shelf.