Critical subwoofer tip

I assume that everyone already knows the importance of phase matching a sub to the main speakers but it’s a little more complicated than simple 90 degrees or 180. The B&W sub that I have has four choices. In every case there has been a definite correct position that can be non standard. My current setup shined at 270 degrees vs the std positions. It’s completely obvious and the other choices would not have been satisfying. 
From my lengthy experience I would want a subwoofer with several phase choices. I personally don’t see how one could seamlessly integrate the mains and the sub without this flexibility. No one asked but i thought this info might be useful to anyone purchasing a subwoofer. YMMV
Good points MC. I have always asked how you phase match a 64 foot wavelength with a one inch wavelength. Can't be done, except at one point at one frequency but you may not necessarily be matched on the leading wave if I'm visualizing this right.. Most will say it's ideal to be in phase at the crossover point but many speakers are not and it wasn't by mistake. 
i certainly don’t know the technical issues involved in matching a single pair of loudspeakers to one subwoofer. I do know this though. If you can adjust the phase setting on your sub there will be a best choice. That’s my experience. 
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.
The idea that reflections negate the need for sub phase matching is erroneous. However, it may be less important for those using DBAs, which (at least from what I’ve read) often run the subs somewhat out of phase with one another.

I have done measurements, and despite reflections, having a sub out of phase with the corresponding main speaker can give a broad and deep (and quite audible) null near the crossover frequency. This isn’t theory or possibility, it’s reality. It’s why expensive subs tend to have continuous phase (delay) controls on them.

phase cohesion is of course important. you want to make sure that the sound from the subs arrives at your ears at the same time as the rest of the music. You will need to adjust the phase to within less than a degree if you want perfect time coherence.

You dont need 5 subs in a small room. One is enough. More subs will take up more space and use more energy. Most of the bass will end up  cancelling the other subs. 

Smaller subs are faster as they have less mass. So they can accelerate more quickly. 

Cheaper subs are made of mdf but if you want the best, concrete cabinets are a must.

The best subs will start and stop instantly. The worst ones will ring like a bell long after the signal stops. 

perfect subs will give you clean bass everywhere not just in the sweet spot.