Granite is good for mass and stiffness but the problem with granite is it rings, which accounts for the problems you heard. What I found works pretty good is to set the granite on a sand bed. Only a small amount of sand, an inch deep is plenty. Mixed with a little mineral oil it packs stable, doesn't scatter, and stays dust free for years. This pretty effectively damps the ringing. Of course you still do not want to put your components directly granite! Its best use is as mass, but mass that retains dynamics not mass that simply damps the way sand does. In my rack its just one element among several that work together to achieve strength, mass, and stability.
My Basis table was on the floor for years. Tried different things but the floor, even an ordinary wood one, is very hard to beat. The rack I finally came up with uses a combination of concrete, ABS, sand, granite and carbon fiber. Its better even than the floor. But not by much. With the factors Tom already mentioned going for it the floor is just awfully hard to beat!
