I have limited space, just picked up a entertainment center. The only place I can put my subwoofer is in one of the lower cabinets of the ent. center. BUT, I have to put it on it's side to make it fit. Would this do any damage to it? I would move the little rubber feet from the bottom to the side to protect it and vibrations.

Probably not. But aren't there any space on the floor for your sub? Subs would sounds (a little) better if you put it on the floor instead of in a cabinet.

Putting a sub inside a cabinet is a HORRIBLE thing to do! You need to experiment with where it sounds best in your particular room but realize that they will give you the best bass usually when placed in a corner. Also remember that the bass from subs are onmi-directional so it can be place behind a couch or anywhere else as well.

Go here, and scroll down to #8 (subwoofer placement). Have fun!!!

I agree with Frank S,

Not only will it sound terrible (all speakers should be on a solid, stable platform), but it will likely cause your entire cabinet to vibrate. Vibration is never a good thing for electronic items.

And while putting it on it's side, MIGHT not hurt it, it was not designed for that. The sound will be effected.