Author of the Audiophiliac blog on CNet, Steve Guttenberg (he doesn't like being asked where Moses Hightower is, so don't try it) has put together a handy little how-to guide to choosing and installing subwoofers in a home theater. As well as covering placement, Guttenberg also advises on how to fine-tune your sub. Useful. [CNet].
@MaxRC: The vibration of the box is in phase with the cone? Really? That makes no sense to me... Force moves cone one direction; equal and opposite force moves box the other direction, only a lesser amount given the disparity is mass between the cone and the box. Net result, however, is that cone doesn't reach full extension, so isn't that damping?
I'll agree with you that coupling something to something loose isn't a grand idea and that walls and floors can be loose, but its almost a separate issue in my mind—loose walls and floors are going to be acoustically coupled to the bass even if not physically, so the solution there is to either decouple the walls from the floor, or couple and reinforce them...
That all being said, my point is that acoustics isn't binary. A little pad isn't going to solve a much more complex problem and too many people think its a panacea. I'd rather deal with the issues by coupling things to immoveable objects than decoupling.