This thread is a leftover from a failed experiment at having a separate circle for subwoofers. There's a lot of great links here so I saved it for reference. Hopefully your post will revive the subject here, who knows.
I've haven't seen any rules of the road for how to set your phase switch, usually it's just either or with a 180° switch with a recommendation to set it by ear, but I've encountered continuously variable phase knobs and wondered how you'd measure to get the correct response. Great question.