X: another thing you can also look at is forming your own bases out of concrete. it is pretty easy to mix your own and add whatever colour you'd like.
Not as nice as Granite but not as expensive either!
I've heard of ppl doing that before (putting their speakers on something dense and heavy). Supposedly the bass is supposed to resonate through the bottom of the cabinet into the floor and cause vibrations in the floor. isloating it like you've said is supposed to eliminate that. BUT, if you have a poured concrete floor I don't think you'll change anything.
If you do put em on spikes as well, make sure you have little divit under the point of the spike else your speakers will walk across the granite! (depending on how loud you're playing your tunes of course lol)
thanks for the correct spelling of his name, damn I'd be embarrassed if I wrote him an email or something lol.