I share experiences with others, whereby adding granite slabs under components or (especially here!) speakers has had a significantly deleterious effect on sound quality. Harsh, bright, brittle sound. Just awful.
Of course there are also known quality products where granite slabs / sheets have worked wonderfully. HRS incorporates granite in their platforms. Acora speakers (granite cabinets) sound awesome. I've got a friend with granite bases (sandwich design) for his Vandersteens - works great.
I think the most common theme for success is that damping needs to be applied over the full granite surface (at least 1 side). Iso Acoustics feet on 4 coroners isn't enough - I even tried that with my speakers and the granite slabs - no help.
Herbie's sells (no idea of current availability) his "grungebuster" material in sheets cut your needed size, and I believe that's what my friend suggested for use in a sandwich with granite platforms. Seems reasonable to me! At least cover the top of each rack shelf, in your case.