Though I'm too ignorant to offer a "valid" opinion, I agree that one would first treat the room as good as feasible. Then the multiple subwoofers would be the "last chance" to fix problems that the treatment did not solve.

In my specific room and equipment found that "trying nearly every setting, even the dumb ones" was productive finding as good possible subwoofer adjustments. There is currently one sealed 18" sub under the right main. Later I want to build an identical twin to put under the left main.

I had another sub in the room rear but currently am not using it because it raised group delay and decay time along with fixing the null. Currently the flattest setting seems rather weird but it sounds pretty good--

The coax horns are crossed with the 12" coax woofers at 1 kHz 24 dB Linkwitz-Riley, and the 18" sub is low-pass filtered at 500 Hz. There is no highpass filter on the mains 12" woofer at all, though the sealed cabs start acoustically rolling off around 100 Hz or whatever.

I would never have theoretically decided to try that setting though it works pretty good. It was only discovered by trying a lot of settings even if it seemed stupid. Kind of "guided stupidity" or "successive approximation". If one stupid setting had surprising results, started tweaking in the ballpark of that stupid setting to see if it could be further optimized.

Which is of course specific to my room and equipment. Inapplicable to any other setup.