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 would try wood by itself as an experiment to see if you prefer the sound. No spikes, just wood. Spikes under any equipment can cause the sound to become lean.

At first thought it seems like a good idea, but in practice granite is a poor choice for an amp stand.

IME granite makes for a drier, harder edged sound with far less bass bloom. In fact, I would say harmonics, the natural decay of the music is vitally disturbed.

Depends how thick the granite is. I use three inch thick granite at are lake system without issue.
Solid state amps aren't generally susceptible to the things there put on.