Setting a sub on stone really accomplishes nothing unless you isolate the stone from the floor which is what I use to do with rubber/cork squares. I found it just as easy to use Audiopoints/discs for the floor and I feel mass loading tends to solidify the bass,,, for my ears anyways. 
Robin
Ahh yes, this is important. When I used the concrete slabs I used racquet balls to float the sub and I also used those 1 1/2x4 inch sample granite square slabs that you see as samples in Home Depot etc... I then applied a rubber backing to both sides (like what's on those cork isolation pads).
So it was:
Concrete slab>Sub>concrete slab>rubber backed granite footers or racquet balls.
It worked well, I thought sandwiching the sub made positive gains. I would have used some spikes but I couldn't find anything cheap at the time that could support the weight which must have been over 200lbs.
In fact I have several pieces of granite left over from projects and I've applied the rubber padding or the cork pads and I'm using that under most of my components, seems to work well enough.