Critical subwoofer tip

OP is specifically commenting about setting up Phase.

With the subs I have, this was the easiest part of the whole process.

Reverse cables on main speakers.
Play sine wave file with same frequency as subwoofer crossover (ie 50Hz). Turn volume up until SPL is 0db with SPL meter set to something like 80db.
Turn sub phase from 0 to 180 degrees one step at a time and the lowest db on SPL meter is the correct phase. I assume if there is a switch 0-180 you would just flip the switch and see which one has the lowest db.
Reverse main speaker cables back to correct position.

Just to be clear, you adjust the sub phase for your seating position, not the same phase as your loudspeaker but I'm guessing it's near the same thing. Unless you are sitting at a null point. Then the sub phase will be different than your loudspeaker. Agree?  

I spent over a month getting my pair of t7i's to even sound like they managed any semblance to bass response. I can assure you if they are completely out of phase with your mains it does make a difference! You want those subs pushing out and contracting at the same time your mains are. I know there's better terminology but I'm listening to Clapton while responding. It was a pain to get it right by ear but you'll be rewarded when you do. If phase wasn't important why would we have the ability to set them? why would there be mfgs designing then with servo auto driven phase adjustments? When I did what REL said and heard when it was audibly louder it was in phase and when you listen again from LP you'll know it. Still fine tuning but getting there and loving the ride!

@golfnutz, great way for adjusting the phase . I learnt it from Barry Oder .
I totally believe in adjusting the phase/timing  to the listening position for the sub and the mains . You can do the same with an av receiver by adjusting the subwoofer distance setting .
I use 2 subs jl audio 113v2 integrated with my magico m3. The rt sub is phase adjusted  to the rt speaker and lt to lt main ,  at the listening position. Changing the phase, the difference is very obvious  as the subs are placed in the optimal position using the crawl method .so the primary listening position is not a null.
I totally agree with the OP .

Don’t see how????
No imagination. Change the vertical and/or horizontal direction of the wave eminence. Avoid down firing configurations.
A phase switch is mostly a decorator option.