Decoupling from the structure is great, but a couple slabs of marble won't git 'r done. At least not very well. If you're dead set on big rocks, you'd be better off with rubber layers between the floor and the marble, and the marble and the cabinet. Many subs come with feet that address this variously... In some dedicated rooms there are isolated slabs or sand traps in the floor.
I think isolating subs from the floor helps you make better judgments, but it might be less fun and impressive for clients, depending on genre. Some rappers rely on resonance and room excitation as a way of deciding when there's "enough" thump. In flat, non-resonating rooms these folks have a tough time with that call. More of a mix issue, but can matter in terms of attendance... the best fix for that is a separate listening room.
