First, what receiver do you have?
Your sub out level on your avr should roughly be set between -2 and +2. If your avr has auto room level/calibration, you should start there. Then use the sub's volume to adjust to your listening taste.
This all should have been done by the professional installer when they setup your system.
M80's, QS8's, M22's, CHT SHO-10, Dual CHT SS-18.1's, Onkyo NR3008, Mitsubishi WD-73740