Critical subwoofer tip

Yes doing anything perfectly is always very hard but with the right sub to fit your system, a sound meter app on a smartphone and streamed white noise, doing it very well is not hard at all.

If you do that versus merely doing it by ear you might be surprised about the difference. 

also the more subs to integrate the harder it will be to get it perfect. That’s basic math. Well enough is always much less difficult.

In my experience adjusting subwoofer phase matching to the mains using a fixed sine wave/null points was vexing. 
It was much too room and location dependent. 
I found a better result in my system by playing a repetitive kick drum and trying to match the beat sharper than softer. It seems more likely to result in a phase structure that is consistent from ‘the other room’. If you have inaccurate phasing, when you go elsewhere in the house it seems much more apparent. 
If you get this kick drum right, it seems more correct. Not just from the listening position that has room issues effecting it, but from a broader area of the structure. 

kick drum or many modern pop recordings with synthesized percussion will do nicely. You want to maximize the “kick”.

there I said something positive about modern pop recordings. 👏👏👏

Anyone familiar with phase cancellation technology used in headphones should appreciate the importance of optimizing phase relationships. The problem is that if you have a two position phase switch on your sub, you can only reverse the phase 180 degrees, and if your phase is only off by 90 degrees, you won’t hear a difference either way. I have two subs, and I accidentally had the phase switches in opposite positions from each other because like a lot of you, I didn’t think it made much difference. When I put both subs in phase with each other, the difference was obvious—I got louder, punchier bass. Golfnutz got it right—if your subs are out of phase with the mains, they’re literally working against the mains bass output, and the overall bass will be reduced. So setting the phase is simple—just use the position where the bass sounds the loudest. Just keep in mind that with a 2 position phase switch, you may not hear a difference.

Worth noting that when choosing source material to tune phase with that is where a sound meter is crucial.  You have to be sure there is bass down in the frequency range the sub is covering.  Many recordings with good bass will not do much below 40 hz or so. So Adjusting phase on a sub with music containing little or nothing in the range the sub is covering will do little or nothing.  Details matter!  🏆