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.

I use a 2" thick concrete patio slab 20" square (cost $3). Up on tiptoes (old stock have had for ?25 YEARS. $28 when new) , with the amp on size ten butyl rubber bottle stoppers on top of the slab.(another $6)
Cheap as can be and works great.

Granite may 'ring' a tiny bit, but it may not, depending on how the amp sits on it. Personally I have never been able to hear a chunk of granite make any sound, or feel it vibrate in any way.

PS if you have to ask this: guess what: IT DOES NOT MATTER because you don't hear any difference anyway.
IF YOU did hear a difference, then you would not be asking...
You are worried about the granite 'in theory' WHY???????
Do you just worry about useless stuff? And guess what.. asking here will get you a dozen different 'theories' right back, Not ONE of them is worth a damn. as they are all just 'theories' too! They can't 'hear' your amp on it's granite stand, only YOU can. Rant over.
(Sometimes it IS neccessary to kick butt...LOL)
And I apologize if the op feels hurt by my rant...

And if the granite ringing in theory bothers you, add a constrained layer of damping material to the bottom of the granite.
A dirt cheap version would be a self stick vinyl flooring tile. just stick on the bottom. problem solved. in 'theory anyway.