I have a history of using Stone inclusive of Granite for a variety of supporting roles in my own HiFi System.
I have used and still use Granite as outlined in the following: Plinths @ approx' 90Kg each under Cabinet Speakers, a Plinth for a Garrard 401 @ approx' 90Kg, a Plinth Under Valve Mono's @ approx' 300Kg and Bespoke Built Rack Shelves @ approx' 25Kg.
I have evolved with how I am utilising the Granite, it has not been used as a sole material for many years. It is used a Tier in a support Structure, and the position it is placed usually means it is a Upper Tier material which needs careful consideration for a footer being selected to be used.
I have never stopped working with materials that can be used in a structure in conjunction with the Granite.
Most recent, the material of Interest and being adopted is Phenolic Impregnated Densified Wood. This material comes in at 1400Kg per Cubic Metre, and measures for Damping and Dissipation in a much more desirable manner than Stone or Metals.
This material has made a impression on myself like few other materials have when used as a part within a Support Structure. The improvements on offer are seemingly tangible.
I encourage the OP to look into this material to produce their design for a Speaker Support, it is finding a place in the construction of Hi End Speaker Cabinets and is being picked up on by other areas of the Audio Industry.