Thanks, Sonnie. An interesting read but of course YMMV, as they say as I doubt I'll get a curve that looks like the one you achieved! But I will start fooling around more with location options when I get a chance w/o the wife in the house. She does NOT appreciate the subtleties on this stuff at all so symmetry is cool, but putting a sub, say, in the middle of the room, to hit that perfect balance would be "painful". Too bad you can't correct WAF with REW and a BFD! Now THAT'd be a product combo.
Dave - the subs I have offer a 0/180 switch only, and I've tried to place them in symmetric location relative to the fronts, and on a plane with the fronts and the center. I havent started to try to measure with the mains on as yet, but in effect I'm doing the "trivial" version of your "alternative" with both subs getting the same signal and in the same phase (assuming they're wired identically, of course). I like your suggestion of matching with each the mains, but the reality will be that I can control phase only for both subs in tandem from the processor. but I can control crossover for each of the 5.1 channels independently.
Thank you both for the food for thought. Will post some results when I get the chance to do more.