Granit amp stands are they killing my sound?

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.

I have tried many different materials under my amps, preamps, cd players and turntables. My 3" granite slab is among the worst sounding of them all, no matter what type of couplers or damping materials are used. I don't believe the problem is as simple as "ringing" because it's hard to believe a 180lb slab of anything is ringing. But the granite does have serious colorations and they are unmusical colorations---they interfere with my enjoying the music.

I recommend trying some different types of wood instead of the granite. Each type sounds different and different thicknesses of the same wood sound different. You just have to try some yourself and reach your own conclusion about what you like best. My bet, however, is that the wood will sound much better than the granite.

A safe recommendation is to use a shelf platform made of 3/4" Baltic Birch void-free plywood. I have found this to be very neutral tonally with good dynamics and detail. Don't be tempted to use 1" Baltic Birch (too dull in the treble) or some type of plywood other than Baltic Birch. And whatever you do, stay away from mdf. Despite lots of companies using it for equipment shelves and speaker cabinets, mdf is also among the worst sounding materials.

Wood might work, but pay special attention to grain orientation, which can be easily heard. Its far worse than ringing. Wood is even more directional than a fuse, and you wont want a grainy sound.