Downfiring subs on carpet?

It is very important that the sub can not move or vibrate. It should be coupled to the floor as best as possible. Spikes are the best whether the sub is on carpet or not. Right into the floor no pads. 

Coupling the subs to the floor would probably tend to increase the mechanical feedback problem, don’t you think? That’s why they invented isolation for subs and speakers in the first place. Hel-loo!

Three spikes please! Four can rock.

Hel-loo, subwoofers moving/vibrating = distortion. Put your hand on your sub as a real low note comes along. Ideally you should not feel anything but all of you will notice your sub vibrating to one degree or another. Geoffkait uses his sub for massages. 
Ideally you would mount your sub driver in a concrete wall. Some new designs like the Magico use two drivers firing 180 degrees from each other. This is called balanced force. These subs will not move at all and if the cabinets are stiff enough you will not feel anything while the sub is playing. The best way to keep most subs from vibrating is to couple them to a larger fixed mass, the floor. The best way to do this other than bolting the sub to the floor is with spikes. This would give Geoffkait an even better massage.

The entire room is moving, even if it’s on a cement slab. Hel-loo!