marble or granite slab under floorstanders?

There is a school of thought that says even with a concrete screed floor the Earth's seismic activity will affect loudspeakers with spikes. Similarly footfall on wooden floors will have an impact. There is an app you can download on to a tablet to measure seismic activity by resting the tablet on top of your speakers. The effect manifests itself in the way musical notes decay, but at my age and with my hearing I'm not sure I could hear the difference.

Clearthinker, doesn't matter how much mass you put under a turntable. It will still pick up all kinds of crap. The only adequate solution is a well suspended turntable (SOTA, BASIS, SME, Dohmann) or a MinusK platform. Electron microscopes directly on a concrete floor are a nightmare. This is usually what MinusK platforms are used for. 
As for other equipment? You can theorize that tubes need isolation although I have never seen proof of that. Otherwise it is just another profit making scam perpetrated on audiophiles. The only reasons to put anything under a loudspeaker are to keep the spikes from marring the floor or to get them up to the right height. 

Maybe these will help you out 
 
 Gaia Isoacoustics Isolators

 https://www.youtube.com/watch?v=eKCWRCE9Vns

Stay away from marble, it causes a ringing characteristic.