What material is based below a down firing sub?

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.

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%)

what nsgarth said about finding the best location for your sub is also my method only after i walk around the room & note where i liked the bass best i also crawl around the room & compare locations.