Granit amp stands are they killing my sound?

Granite is reflective as a polished hard stone. It can cause ringing. I would recommend a wood layer that you can drain vibration into via spikes. the wood with spikes is a common stand type which is fairly cheap but constrained layer damped shelves are also good but more expensive choices.

I never suggested that the Granite slab would resonate and ring like a bell. What I meant was because it is a hard polished like glass it may reflect/feedback and cause other objects to resonate. I don't know if this is universal, but it seems quite plausable. Mirror like surfaces are best avoided in audio land, that's all.