Critical subwoofer tip

Here’s some REW measurements I did today showing how changing the phase on the subwoofers effects frequency response.
This isn’t about how good/bad the subs/mains/room are.
No changes for volume or crossovers on the subs, phase only.
Descriptions in the images should be self explanatory.
The first image are the speakers, and basically my best attempt to get the frequency as flat as possible without using and EQ or DSP. The results are a combination of crossover/loudness settings and room position.
I tried adding a third sub yesterday, and couldn’t improve on what I have with two subs. Mind you, it’s only an 8"/150 watt sub, compared to 15" and 900 watts.
https://imgur.com/a/YJeNzUh

Setting the correct phase is much more important with a single sub, and less important to not important at all as you add additional subs.

Does it also become less important the lower you set your sub's roll-off point?  For example, I have two subs set to roll off at a pretty low 40Hz because of the low frequency extension of my main speakers.

@mitch2 

If you're subs are hitting any frequency same as the mains, then it matters. It's either in-phase or out-of-phase, that is, the sub drivers are moving in the same direction as your mains. When they are out of phase, you run the risk of having frequencies cancel because the drivers are moving in opposite directions. There's lots of illustrations on the internet showing this.

Is there another way to figure out the optimum phase except to listen?  Maybe use an SPL meter at the overlap frequency, play the same program material at the same volume setting, and then set the phase at whichever setting achieves the highest SPLs?

Your suggestion is how lots of people try to figure out where to set the phase.

TAS suggests doing what you’re saying, but reverse your speaker cables so your mains are out of phase first. Using an SPL meter takes any guessing out of the equation, it's that obvious.

Here’s the reasoning sent by Rythmik in their setup sheet:

"Here’s what’s happening when you follow this procedure. By reversing the polarity of the main speakers, you’re putting them out of phase with the subwoofer. When you play a test signal whose frequency is the same as the subwoofer’s crossover point, both the sub and the main speakers will be reproducing that frequency. You’ll hear minimum bass when the waves from the main speakers and subwoofers are maximally out of phase. That is, when the main speaker’s cone is moving in, the subwoofer’s cone is moving out. The two out-of-phase waves cancel each other, producing very little bass. Now, when you return your loudspeakers to their proper connection (putting them back in-phase with the subwoofer), they will be maximally in-phase with the subwoofer. (It’s much easier to hear the point of maximum cancellation.) This is the most accurate method of setting a subwoofer’s phase control. Unless you move the subwoofer or main speakers, you need to perform this exercise only once"