I cut some high density packaging foam and placed it under my sub, raising it about 3". It did 2 things, reduced the floor vibrations (good for neighbours) and it made the bass sound less boomy (which may be a direct consequence of reducing floor vibrations). The overall effect was almost like the sub's gain had been reduced.
Listening to music with and without the foam, I liked the sound better with it present. In movies, comparing sound alone the difference was less apparent.