For a quick, but noticeable, change in subwoofer response and isolation, the GRAMMA (also known as the "Subdude) does the trick. I've owned one for over a year now and I'm still impressed by how it helps isolate the sub from my floor. Proper acoustical treatment is a good path to take, I also especially like the realtraps product, but don't mistake taming reflections for acoustical isolation. The GRAMMA actually isolates my sub from my floor, noticeable in reduced vibrations in neighboring rooms. It DOESN'T reduce actual bass level except to eliminate any items in the room the may be vibrating along with the sub.

For $60, I've found the GRAMMA to be one of the best tweaks yet for my system. It does exactly what it says it's going to do.