Are you using a SPL meter and test tone to measure the levels? Typically, you would set your subwoofer level at least 4 db above the level for the mains because human hearing is less sensitive in the bass range, and probably add at least 2 db more if you're using the Radio Shack SPL meter (the sensitivity of the mic on that unit drops off below about 100 Hz). If it sounds right to you, then that's where you should leave it.