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.

Nsgarch is off base here. First, the carpet makes zero differnce in a performance standpoint! Infact, the carpet may benefit, in that it can help hide higher frequency distortion or overlap/crosstalk!. Bass waves in play here are so big, that any carpet depth laying over a hard foundation, is going to affect NOTHING, in terms of intended performance! That is fact.
Second, anchoring a downfiring subwoofer with spikes, as long as the sub is heavy enough, is fine in carpet. Actually, the woofer is moving against the weight that's it's secured to from the box above it! As long as the woofer is pretty stable, and even at that, you're not going to have any downsides on carpet. Placing a down firing woofer on a marble platform is not really going to improve here. Infact, unless the marble platform is exponentially heavy enough, it's probably not as good as spikes into carpet/subflooring.
I've heard many people worry about downfiring sub on carpet, or otherwise. If you wan't to know, just experiment and find out!

I agree with Frnlamb in the sense that I tried various stuff -- slate, tile, glass, pavers -- under the subs (used with my former speakers), as well as various spiking arrangements and frankly couldn't tell the difference. Ended up firing directly into the carpet, which was on a concrete floor. Dave