If you want to hear the difference without spending a lot of money, you can just pick up a couple of concrete pavers from your nearest landscaping center. Granite is more dense than concrete, but the effect is much the same. If you like the result, Granite is the way to go.
Hey Beek. The platforms underneath the Vistas are Auralex SubDude II isolation pads. Their purpose is to reduce/eliminate bass transfer through the floor into the neighbors' unit. They do work well to decouple the speakers from the floor, but I definitely prefer to have them on granite. They don't sound bad, but the bass has lost some of its' punch.