one suggestion, if you can live with it, is do what i did and cut the carpet completely out from under the speaker. I then took hard wood boards and ran them across bolting them in to the floor joists for a really strong and solid platform. Then i set the speakers feet right on those boards.
I used the brass feet from Mapleshade to great affect, but now, like a lot of the guys above, use the GAIA feet under my Thiel 3.7 speakers.