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FWIW, I did a sweep of the sub by itself and a sweep of each individual main.
I then went into the OVERLAY Phase window and placed the vertical cursor at the SUB/Mains XO point (my case 40HZ).
I then enabled FDW (Frequency Dependent Windowing) and removed smoothing.
I then wrapped and unwrapped all phase plots (mains and sub) so they would be replotted and aligned at the Sub/Main XO point.
I then read the phase of each speaker at the XO point and then used that information to adjust the phase on the sub.
I put the sub's phase half way between my left and right main's phase (split the difference).

This gave me more control than just +/- 180 degrees. The sub is now within @ +/- 8 degrees of each of the mains.

You can then verify the results with a combined sweep to see the summing/nulling/smearing at the XO point.

You can also repeat the procedure to make sure the sub's phase moved in the direction and amount you intended it to (since the sub's phase knobs are usually coarsely marked).

There maybe better ways of doing it, but this is the procedure I used.

I like the methodology you described. but I am not sure exactly HOW you managed to do that with REW. Any chance you could give me a more detailed step by step rundown?
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