Sorbathane under granite slabs for speaker bases?

What Herbie says his products do, they do .

In my all-Naim system the best and the cheapest upgrade was Mana Acoustics stands for SBLs. Speakers sit on a light platform which is "floating" on the set of spikes going from Mana frame to the platform, and four more spikes face the floor. So, I cannot figure out if SBLs are coupled or decoupled from the floor, but the end result is truly magical!!!  Heavy granite slabs just lower the resonant frequencies, something I am trying to avoid in my professional business when doing microsurgery under x200 magnification.
My say: four spikes from your slab toward the speakers, four more toward the shelf. Then substitute granite slab for Mapleshade piece of maple. 
Herbie LP mat worked marvels on my Sondek, also keep in mind that I am seriously biased towards Naim/FlatEarth sound... ;-)

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