Downfiring subs on carpet?

It doesn't cost much to try both.  Why not compare and report back?

Jon, That's my exact thinking. Due to arrive on this coming Monday.

The floor construction matters not so much the rug.

I use subdude on my upper levels with suspended plywood floors that will vibrate and muddy up the bass and obscure the midrange with most any sub. I also use them there under my bottom ported Ohm Walsh speakers for same reason.  

In basement with solid concrete floor foundation and dense but thin carpet and pad, not needed.

Anyone here recall the Stereophile reviewer who couldn't figure out why his system sounded funny until the cleaning lady confessed to replacing his rug with the fibers running the other way? Now what was his name? Oh yeah! Jonathan Scull! Also something about the torque of the screws on his CDP. Or direction? Something like that. Whatever. He was the champ. Until now.

Mapman is correct. 100Hz has a wavelength of 10 feet. The only way the carpet would effect the sound is by lowering the resonant frequency of the floor by adding weight but no stiffness. The trick with the floor is to get it's resonant frequency outside the range of the subwoofer. Concrete is the ultimate. You can get wood floors stiff enough by using larger laminated joists and doubling up on the plywood. Use construction adhesive on everything. What you put the woofer on makes absolutely no difference. My woofers are on concrete. If I play a 20 Hz test tone the whole house becomes a symphony of rattles. Fortunately music masks the rattles. Low frequency sound is very powerful. In terms of distortion the most important factor besides the driver itself is that the enclosure is stiff and very heavy. Ideally when you put your hand on the sub while playing you should feel nothing. No vibration at all. Having said all this you can negate most of these problems with room control as long as they don't boost any given frequency more than 5 dB or so. After that you start running out of power and digital headroom.