Try using a very dense wood that has a totally different sound signature from the hard granite in conjunction with the granite. I've tried many different types of materials for a rack. I have been using a pair of Sistrum racks from Star Sound for many years. Anything that is placed on it immediately sound more live but without ringing or any other sound issue. I also use RTS couplers to clamp each component to its shelf. The combo is about twice as effective to improving the SQ as the Sistrum rack alone. I was very lucky to have a former owner sell me all his Sistrum racks used at very good prices. You can even install this racks between the speakers with little or no drop in soundstage info. My two racks are about 18" behind a line between the two speaker fronts. They allow so much air to circulate w/o ringing vibrations behind heard it's wonderful. Used they are MUCH MORE affordable. Thanks Al Jennings.
Bob