I'd been quite interested in placing marble slabs under my speakers, but found it difficult to source reasonably priced marble/granite, and was also warned-off by several Audiogon members.
Members concern in general was the ringing problem, as has been mentioned here (including how to avoid it). However I was surprised and concerned to hear one member stating that the whole notion of “mass loading/active damping” was “flawed” (although he didn’t put it that way). At the time, the member and I were discussing mass loading of any equipment – sources amps or speakers.
To quote the member directly "a heavy stand... would result from mass loading... will alter the structural Q of the floor to lower frequencies, akin to placing an anvil atop a trampoline." "Once energized (inevitable, beit by footfall or passing car), it will continue to drone much longer and with a great deal of power.” This suggestion made sense to me and really put me off of the marble thing. I actually started giving serious consideration to products like the Townshend Seismic sink/stands.
I’d like to know if anyone is attaching spikes/cones to the marble, then placing the speaker stand spikes/cones on top of the marble slab – ala Mana Acoustics turntable & equipment isolation stands.