I have this on my Velodyne sub and it is set to zero. Everything Marty said is the true in my experience. When I set the phase to 180, the subwoofer bass is much less and everything sounds a little bit out of tune. Whichever setting produces the most bass output should be the correct setting.
matching amplifer output phase to subwoofer phase
I have a PHASE INVERTING amp by (jeff rowland), the subwoofer I am using (paradigm seimic 12) has a phase adjustment knob, is it logical to invert the phase knob 180 degrees from O to match subwoofer phase to amp speaker output?