in my experience with subs..you need to keep them as Stable as possible such that they are Coupled to the floor. I am sceptical about Rugs though..granite under should also be good. Granite over the sub increases the mass and hence couples it well to the floor as well.
last thing to put is a rubber mat under it ! you can put the rubber mat above the sub and sandwiched between the granite if you want...it makes for a constrained layer damping whicn is very good
in the end positioning of the sub is far more important than damping it...if you can "Couple" it to the room acoustically you will get fantastic results...else it will end up being a boom box