It is likely that Soapstone is denser and will ring less than granite would, and thereby make for a better plinth. I used Corian for my TT because of it's propensity to be well damped not ring at all. Here's a pic.
http://cgim.audiogon.com/i/vs/i/f/1231696007.jpg
http://cgim.audiogon.com/i/vs/i/f/1231696007.jpg