Distributed Bass Array configuration

REW measurements definitely show a difference. I guess it's up to each person if it matters or not.
Here's some example of in-phase vs out-phase graphs. Scroll down in the link to view all 5 screen captures.

First graph is comparing dual subs in and out of phase.

Next 2 down include the phase graph for dual subs in and out of phase.

Last 2 dual subs for the Group Delay for in and out of phase.

https://imgur.com/a/RSQiBMi

From REW author.

The vertical lines for the phase graphs are called Wrapped Phase:

" Phase is cyclic, due to the periodicity of the sine function. When it is increasing and reaches 180 degrees it wraps around to -180 and continues with -179, -178 etc. When it is decreasing and reaches -180 degrees it wraps around to 180. The vertical lines show those wrap points. If you look in the graph controls you will see a button to "unwrap" the phase, which allows it to go to 181, 182 etc rather than wrapping, but you can end up with extremely large value ranges in unwrapped phase, partly because any time delays create a frequency-dependent phase shift that gets ever larger as frequency increases.

All that changes at the wrap points is the way the graph is drawn. Think of it more like reading the second hand of a clock, after 59 secs it starts again at 0 and counts up again, but time has not jumped or done anything other than progress as usual. The part of phase that gets plotted when wrapping is used is like showing only the seconds of a time."

Group Delay
" The Group Delay at a frequency is the slope of the phase at that frequency. It provides an indication of how the envelope of signals at that frequency is delayed in the course of passing through the audio chain - if everything is delayed by the same amount the shape of the signal is not changed, it simply arrives later, but delays that vary change the shape of the signal.

Anything that affects the phase will also affect the group delay, which includes crossovers, the response of drivers, the low and high frequency roll-offs of the various parts of the audio chain, any time delays in the system and the filtering effect of the room, its modal response and the frequency-dependent absorptions of its surfaces and furnishings".

Why is there never any mention of level matching these DBA systems with the main speakers using an SPL meter? Otherwise, wouldn’t it just be guessing what the volume level should be on the subs?

I agree, using the 'throwing darts' method isn't the way I'd install a $3200 subwoofer system.

You're twisting it around MC, the whole purpose of level matching is to prevent you from trying to figure out what the volume on the subs should be. That's why its called level matching.

I agree, level matching the subs to the mains is a good starting point, especially from someone installing their own subs. Where you end up doesn't mean the same position is necessary. At least you'll know where you are with reference to increasing or decreasing gain. Each time you add an additional sub, the SPL increases. So if having 4 subs (or more) in a room, I would think level matching becomes even more important, especially, as you said, if you have a difficult room.

Yes, Duke agreed with me suggesting level matching as a good starting point. Thanks for pointing that out.

Sorry MC, you're a dart thrower, here's what you said. Guaranteed your system would sound better if you learned to install it correctly with the right tools. You're the one adjusting the bass whenever the next 'more bass on this song, better turn those subs down again'. If you learned how to use the tools you wouldn't need to do that.

" What I did, once the initial phase is done, was listen to a lot of different music at my normal levels. I abhor meters. Most of the time I listen at a nice satisfying volume level. Sometimes not that loud. Occasionally I listen Supertramp Loud. Gradually, over time, I make really small level adjustments. Really small. Like I turn the knob but about half the time not really sure it did anything. This works because the meaning of the equal loudness contours is we go from being relatively insensitive to bass at low volume to hyper sensitive to bass at high volume".