Improving a stone rack

+1 on IsoAcoustics under your gear.  Probably the easiest and most effective solution for any vibration issues your gear may be subject to.  Next, I agree that if your rack is between the speakers it can contribute negatively to the sound.  Consider yes, thick cloth but also short acoustic panels on the side and behind the rack itself.  Also, given the angles, room treatment on the ceiling between the speaker and listener is always a great idea.

I have a granite amp stand. I found it effective to place vibrapods (there are many alternatives) under the granite to isolate the slab, then springs between the component and the slab. So, micro vibrations do not get transmitted into the slab from the floor and the component is isolated from the slab. Obviously a vibration platform would be even more effective than the springs. 

I usually improve stone by getting more stoned. Works for me great!

I tried a granite slab under my turntable a few years back and it had a considerable negative impact on the sound quality.  I tried it in three modes—alone on top of the concrete block pillars (see virtual system), on top of, and below, a 3" slab of maple. The poor sound was present in all three modes. It was a relief to remove it. My back was not happy. I was amazed at how poor it sounded. 

I have experimented with isolation materials. Hard surfaces like granite, glass, etc is going to emphasize the treble freq. Soft material IE Vibrapods, sorbothane will soften the highs but often at the expense of clarity. Muddy sound come to mind. A tone wood IE maple is the best I have found. I've also used DIY roller ball  type footers with good result. I recently changed from those to springs and saw no degradation to the sound.