Critical subwoofer tip

@larry5729, you really should start with level matching your speakers to your subs instead of just turning down the volume where you can barely hear your subs. At least this will give you a much better starting point for where the volume should be. You've spent all this money on subs, and you don't really know if you have them set up correctly. It's a simple process, and you only need a SPL meter and sine wave signal (free on internet).

@zappas, curious what sub do you have, and did you use SPL meter?

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.

REW disagrees with what millercarbon is saying.

In fact, phase, crossover, loudness, and room position all have an effect on getting the best distributed bass out of the room.
Crossover and loudness don’t necessarily all have to be the same on any of the subs to achieve best performance. It takes several hours of testing and tweaking to make this happen (within the parameters - not too high or low for best integration with mains). This is without the aid of DSP.

@newtoncr - you are correct, measurements should be from listening position, thanks for mentioning this. Also, I should have mentioned, the subs and main speakers should be level matched first.

@audioguy85 - I get it, there's some guy's that don't think subs belong in an audio system. I can respect that. However, adding subs isn't just about hearing sub bass, with multiple subs, and careful setup, you can get a much better distributed frequency response in the bass region without using an equalizer, or DSP. 

@mapman 

Level matching and setting the Phase are the simplest, providing your subs have the functionality. It's everything else that makes it more difficult in my experience. Room placement, selecting the correct crossover(s), and loudness (within the parameters of level matching) become much more harder when you start taking measurements. I tried adding a third sub the other day, and after spending 4 hours of trying to improve my frequency response, I was only able to match, never improve it (most times it was worse). So, I disagree, adding multiple subs and selecting what ever crossover sounds good doesn't mean it's better, you'll only know that if you take measurements. So not so black in white.

@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.

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

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"

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.
https://imgur.com/a/aXlCoXh