Improving a stone rack

I have used Granite for many Years as Sub Plinths for Audio Equipment on Racks and as a Cabinet Speaker Sub Plinth.

I find I prefer the sonic that has been created when using it, when the Granite is a Sub Sub Plinth or a Sandwich Material in a Build up of Materials to create a Plinth or Sub Sub Plinth.

For Speakers I always uses a Spike Coupling and Pneumatic Footer to create some suspension.

On the equipment Rack, I always use Cork as the Isolation footer for the Granite when in a Sandwich or as a Sub Sub Plinth.

Other Materials used in conjunction with the Granite will have  a selection of Footers tried out to discover which tidies up the sonic to a standard I am happy to experience.

Today, my usage of Granite is seemingly being superseded by the use of Phenolic Resin Densified Wood Board Materials, such as the Board Materials from the Brands Permali or Panzerholz.

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.

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

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. 

+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.

Depending on design, you may be able to place sorbitans spacers (buy sheets or strips) between the granite and whatever is supporting the granite. Then do the same between the supports and the floor (cover the sorbitans sheet that touches the floor with felt to protect the floor if the floor is wood).  

I built a "rack" from three 20x40 inch black granite "shelves" and wood supports (hard maple 2x4s glued together for the uprights and 2x4s for the cross supports, the granite is "inserted" into slots in the wood.  Everywhere the granite touches is covered by the sorbitan, and the upright supports have sorbitans on their bottom surface covered by the felt.  I choose the thickness of the sorbitans to not only support the weight, but for the harmonic frequency that needed to be absorbed.

Incidentally, different granites have different harmonic frequencies.  Apparently black granite is the best for audio applications (eg see Acora speakers).

No "ringing", no vibrations, incredible stability.

Having a little fun: Maybe not grandma's tablecloth, but you get the idea. Keep your stability, add some softness on top

springs