Critical subwoofer tip

I assume that everyone already knows the importance of phase matching a sub to the main speakers but it’s a little more complicated than simple 90 degrees or 180. The B&W sub that I have has four choices. In every case there has been a definite correct position that can be non standard. My current setup shined at 270 degrees vs the std positions. It’s completely obvious and the other choices would not have been satisfying. 
From my lengthy experience I would want a subwoofer with several phase choices. I personally don’t see how one could seamlessly integrate the mains and the sub without this flexibility. No one asked but i thought this info might be useful to anyone purchasing a subwoofer. YMMV
Anyone familiar with phase cancellation technology used in headphones should appreciate the importance of optimizing phase relationships. The problem is that if you have a two position phase switch on your sub, you can only reverse the phase 180 degrees, and if your phase is only off by 90 degrees, you won’t hear a difference either way. I have two subs, and I accidentally had the phase switches in opposite positions from each other because like a lot of you, I didn’t think it made much difference. When I put both subs in phase with each other, the difference was obvious—I got louder, punchier bass. Golfnutz got it right—if your subs are out of phase with the mains, they’re literally working against the mains bass output, and the overall bass will be reduced. So setting the phase is simple—just use the position where the bass sounds the loudest. Just keep in mind that with a 2 position phase switch, you may not hear a difference.
Worth noting that when choosing source material to tune phase with that is where a sound meter is crucial.  You have to be sure there is bass down in the frequency range the sub is covering.  Many recordings with good bass will not do much below 40 hz or so. So Adjusting phase on a sub with music containing little or nothing in the range the sub is covering will do little or nothing.  Details matter!  🏆
I won't criticize anyone who sets theirs subs up by ear. Everyone has their own way to do things. That's all part of the fun.
However, I think it's kind of like watching a car drive past and guessing how fast it's going w/out using a radar gun.
I spent a couple more hours today tweaking my subwoofer(s) setting to try and improve the frequency response. There's no way I could have gotten to where I am without the aid of REW.
I'm pretty happy with the first image, as it's only out by a couple of dB (in one place) from what's acceptable using EQ/DSP (5dB).
Second image shows my previous setup compared to today's changes. It also includes how the 'Mains Only - without Subs' measure.
I found a better result in my system by playing a repetitive kick drum and trying to match the beat sharper than softer.
It depends on how the kick drum was eq'd. For some music, there is no low end at all and other's next to no point. Additionally, we often flipped the foot phase on pop recordings.

My favorite piece by ear is a pipe band recorded out of doors. When the phase is correct, focus dials in just like a camera.
Everyone rightfully cites REL subs as having high-level inputs---the sub taking it’s input from the output of a power amp. Often not mentioned is that the Rythmik subs having one of the company’s PEQ plate amps provide both low (line) -level and high-level inputs---low-level on RCA jacks, high-level on speaker cable binding posts. Rythmik’s XLR model plate amps forfeit the high-level inputs.

Rhthmik plate amps also provide a continuously-variable phase control, which is labeled "Delay". The knob can be placed anywhere between 0 (no delay) to 16ms (roughly 180 degrees of phase rotation). Delay can also be achieved by physically moving the sub, but the delay control allows one to find the optimal placement in terms of room modes, the control then aligning the loudspeaker/sub electronically.

If the woofer of a loudspeaker and the driver of a sub are both reproducing, say, 80HZ, and the 80Hz wave from the two are 180 degrees apart in phase (opposite polarity), the result will be cancellation centered at 80Hz, resulting in a deep hole in frequency response. It is no different than the tweeter and midrange driver of the loudspeaker being out of phase with each other, except that with tweeters and midrange drivers in a loudspeaker, the designer is free to use x/o parts to boost the output of the drivers to achieve a flat response.