Sorbathane under granite slabs for speaker bases?

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