Is a down-firing sub into carpet OK?

It works fine for me (REL strata), and I've had the sub on carpeted slab, and carpeted suspended wooden floors.

Be careful with raising the sub, as the sub may be designed to load the room very close to the floor. When I tried it with mine it sounded worse. (As did spikes)

If you can get a cheap slab of something then I'd give it a try, but I wouldn't necessarily expect an improvement.

My REL Storm III is on a carpeted floor (carpet on concrete slab). Sounds fantastic

My HSU VT12? sounds great with 3 inch spikes onto carpet

Contrary to popular belief, downfiring subs are fine on carpet. Bass waves are so long, the thin carpet (relative) is not even a factor to bass response. Infact, firing into a carpet (port as well), if anything, would elimiate some high frequency distortion if at all present. Downfiring designs have this advantage. Still, your main concern with any bass woofer is location and coupling to the room, in relation to where your ears are located.

Thanks for the heads up.

Absolutely! It's a highly effective way to scare the carpet critters out of the house.

If anything, I'd put the slab of marble "on top" of the sub! That added weight should make the response stronger, tighter, and more solid...if anything.