I turn my sub up a scoach when watching movies, and back down when listening to music. I found that if I turn it up too much for movie watching the center channel gets muddied up. Try running it a little louder and you'll probably hear the same effect.

Sounds like the HSU is plenty of sub (maybe overkill?) for your room. No big deal though - have you ever heard any guy complain about having too much power in anything (cars, boats, motorcycles,, etc...)?