You should let your AVR deal with crossover frequencies. That is something you want to set and forget on the subwoofer amp panel controls. What AVR are you using? If it has bass management at all, or if it was made in the last 15 years, you want the AVR to deal with it, so set the crossover switch to 'out'.
As for subwoofer volume, you also want to control that with the AVR. I would set the subwoofer's volume knob to somewhere between 9'o'clock and 12'o'clock, and then control the subwoofer volume from the AVR's 'LFE channel level' on its remote control.