Some say it can cause your floors to "sing" along with the sub, due to the resonations. This is not necessarily a bad thing. My hardwood floors are carpeted, and I'm using a stone slab under them (I Have two subs, both downward firing), and I can feel the floors still "singing " along. You have nothing to lose by trying a rug or a piece of carpet sample under your sub, and let your ears decide. Not sure what that will do for your sound, but if there is a difference and you like it, go with it. There is another thread here about subs, where we were talking about a similar subject. I'm sure somebody will post the link.