What material is based below a down firing sub?

I don't like down-firing subs over carpet, but a lot of people would say otherwise (to put it diplomatically). But if it must be on a carpet, just make sure it sits on spikes or cones that go all the way through the carpet and bite into the hard floor underneath.

Putting a hard platform on the carpet first will work (better IMO), on conditions that:

1. It's at least 16" bigger than the sub all the way around (to properly launch the wave) and
2. It's heavy enough (with the sub on top of it) to compress the carpet enough so that if you step on one edge, it won't tilt AT ALL.

Horseface, before you give up on your sub, try this simple test: Place the sub at your listening position (I know, I know) then with some good material playing at upper moderate levels, walk all over the room and note the spot(s) where the bass sounds best. Then relocate the sub to one of those places. 90% guaranteed (OK 85%)