Look into Herbies Fat Dots and/or feet. I used the Fat Dots under my monitors on a granite slab and got great results.
Moving the speakers upward not only changes the interface of the woofer with the floor, ergo the frequency response, it changes how the high frequencies reach your ears and reflections off the walls and ceiling of your room.
You didn't mention why you thought granite would do something good, sonically at least, but using any other materials to lift your speakers say in blocks with 1/2 inch increments might solve your sonic conundrum when you hit the right height. Then you could go back to appropriate sized granite blocks or any other suitable materiel to get you the desired result. Personally I use hard(er)rubber to block the transmission of vibrations from or to my speakers and keep them as thin as possible so as not to deal with the issues you have encountered.
I use granite pillars for our small surround speakers, wife would not have anything else in the living room; to modernistic, too techy,doesn't blend well with the decor. I have to admit, the granite pillars with small bookshelf speakers looks great and becomes part of the decor while serving a purpose. Not a granite plate, but an entire granite pillar. It works great.