I use these under my subs with great results:
http://www.amazon.com/gp/product/B001140OZ0/ref=ox_sc_sfl_title_3?ie=UTF8&psc=1
http://www.amazon.com/gp/product/B001140OZ0/ref=ox_sc_sfl_title_3?ie=UTF8&psc=1
What's under your sub?
Some sub manufacturers recommend placing a down-firing sub on a slab of marble/granite when the listening room is carpeted. Has anyone had any luck with that? I tried using a granite slab under my REL Storm III and found that the bass was much cleaner with the sub on the carpet and not on the granite. I have carpet over a concrete slab. Don't know if that made a difference.