What material is based below a down firing sub?

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

Subs radiate in all directions because they are only extreme LF bass frequencies - it makes little difference that it is down firing or not...as long as stuff is not rattling (floorboards) or the sub is on an uneven surface and wobbling then you are probably ok.

Down firing is actually an advantage as you are less likley to damage the woofer.

I called Vandersteen Audio with the same concern and got a response very similar to what Flrnlamb said, but the only downside to not putting it on carpet mentioned was the possibility that the speaker would be more capabale of movement ("microvibrations") if not solidly tied to the floor by spikes.
If you put in a platform that the speaker can get a real good bite on, and the system is very well anchored by weight as Nsgarch suggested, seems like it shouldn't degrade the sound.

I actually went trough a "floor upgrade" with the same sub about a year ago. I went from carpet to hardwood floor (real hardwood). To date I can’t say I have noticed any substantial difference with the subs performance. I have to be honest ... I was hoping it would be dramatic - but no. If anything - it seemed to add a lot of echo that creates a whole new problem to deal with on the upper frequencies.

I'm running the Sig S4's so I need all the bass I can get. I thought the sub would be a huge performance increase but I can’t get it to match up right for my taste. I’m kind of a bass junkie but I think my destiny lies on a different path. I'll keep the Seismic for movies it's a brute. I dont think you can go wrong with it if you can get it to match up with your speakers.

I wouldnt listen to us however - go to a gravestone shop or a shop that puts in counter tops ... see if they have some reject marble or better yet granite. I bought a 1" thick piece once for a different project for $15 it was 24" square but a little marred up. Give it a try - what the heck. Bricks, cinderblock, concrete squares ... it's cheap and worth a test.