Using Granite under speakers

I am considering using slabs of granite under my Von Schwiekert VR4SR speakers which currently sit on spikes on discs on suspended wood floor. Have been reading that such arrangement as I have now can bleed bass into the floor and reduce bass impact and definition. Any one with experience with using granite like this? And do you then use spikes under the speakers directly onto the granite? How thick does the granite need to be for best effect?
Thanks
Joe