Critical subwoofer tip

@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. 

My sub has continuous level crossover and phase adjustments. It is right next to my listening position so very easy to adjust properly. Phase matters the least of the three but still matters and makes a subtle difference on the sound. Granted it is not easy to get tuned in in many cases especially where sub cannot be adjusted from listening position.
The more subs the more randomized the wave patterns are in the room which is a good thing and playing with phase an exercise in diminishing returns.
Funny to me how everything is a black or white issue. Phase either matters always or not at all.   Even fuses:  same deal. Yardda yadda yadda. I know. I know. Thinking is hard. Sometimes I try to think but nothing happens.

@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.

Agree about the measurements needed to do it right and get full value from a sub. I use a simple sound meter app and white noise for level and crossover but not enough alone for phase. Have to tune that by ear and only after the other two are dialed in. YMMV.

Good thread on a tough, often debated topic.  MC correct about ability to sense phase of say 20hz and 40ft wavelength using his numbers but two points.  First, thats the extreme case. For most folks xo is up say at 60hz.  So that means a wavelength of 13 ft  for room length.  At 80hz then 10ft is the length. That's real world and pertinent at the top of the sub's range.  Run in phase then, if overlapping some with mains, will produce a peak or boom (best not to use node here)  that will be corrected if the phase is rotated.  At 180 degrees it's a notch and we aren't bothered as much by missing freq.  So yeah, while MC seems theoretically correct at the lowest frequencies, in a modest room at a typical xo point, it could matter.  The OP didn't mention his room size.  The bigger, perhaps the more phase indeed matters.

Second, and this is a question that pertains to rotating the phase, but if a first harmonic is above the sub xo, say 120hz and is therefore played by the mains' low drivers, does our perception treat it as inferior to an in-phase harmonic?  This has prob been addresses ad nauseum in the speaker builder's threads for mid to tweeter crossover design.

Prob over my head now so I'll just sit back, spin some music and see what the group thinks.