Good idea there, let me try maybe some blu-tak between the granite and the cabinet surface, and blu-tak between the speakers and the granite. i wanted to actually get 1' square MDF boards to place between the granite and the table, and i am also considering getting hold of a couple of telephone directories to keep the speakers on. The audioengines showed marked improvement when placed on a couple of heavy books.
I tried using MDF board - it does not help in any way. While MDF may be acoustically "dead", not having a a 100% immobile platform fastened to the supporting structure may actually introduce another source of vibrations. Reducing contact area with spikes or 5 spots of stick-on (centre+4 corners) may be the best bet.
Cheers