At bypass ... both the sub level and crossover is by-passed so it doesnt work so I set both to 0 or lowest level. But in the speaker level of my receiver my sub is set to like -6db but my sub is really big for the small room that its currently in. In your case it may differ. So when you balance all your speakers at 75hz or whatever level....your sub should also be as high as that level so adjust your volume level till your sub level is as high as your other speakers at the same level.

http://www.quadloft.com/hometheater- new HT