Critical subwoofer tip

It is not just Phase but Time also. The sound of the woofer has to get to your ears at the same time as the sound from your main speakers. The only easy way to match both is digital subwoofer management. You will never get it dead on without it. It takes the sting out of subwoofer matching.

I have to agree with Erik and others.  The room is the elephant... in the room.  At freqs being  discussed the room resonates in many modes at exactly a sub's typical range.  Doing SPL is likely swamped by this.  Some cool links out there that show the locations and freq of a room based on dimensions.  It's humbling. DBA?  Place subs at room nodes.  

I believe timing delays are meant for a sub placed at a distance different than the mains but prob more importantly,  different than another sub.  The one further would need the delay so the behave coherently.  I'm not sure about the EE implementation in all circuits but phase does not ideally equal time delay. Phase change is applied to all freq equally. Timing is a shift that can look like phase change but not equally.  Since freq is 1/time period of a wavelength, a simple delay in timing is therefore freq variant.  A 20hz wave is 50msec, 100hz is 10msec.  Applying say, a 10msec shift is like opposite polarity for 100hz but is less than a quarter shift for 20hz, or equal to less than 90 degrees.   So a sub placed near mains or equally distanced to listening position really shouldn't need timing.  If timing improves things in this freq range, I'm putting money on effectively de-tuning a sub-mains XO bloom or notch, or a room node.   That's not at all implying it didn't work.  But then this is why around 90 deg will usually do this, isn't freq variant, and works for the other bobbles in the xo area equally (but not room nodes).   

Delay is to achieve the same impulse arrival time at the listening position. 10ms is about 11.25 feet. Delays around 1ms are more common.

An All-Pass Phase control does not affect all frequencies equally. It is used to get the best integration between the non-linear sub and main phase responses. see  PhaseControl.png (868×506) (ielogical.com) for an example

...and then there's the simple solution of delay for time resolution of waveforms.  Phase matching is easy....

I recently purchased a sub to augment the bottom end of my Spatial M3 open baffle speakers. Due to domestic constraints, one sub is all I will have in the living room. 
The sub is front firing, and placed about 18” from the rear of the Spatial. If you follow me here, I was concerned about the back wave (open baffle) cancelling out the sub’s forward wave. I made all my adjustments- xover freq, volume, Q, listening mode (Eq).
Things sounded very good- then I flipped the phase to 180 degrees.
This caused things to snap further into focus- I imagine the back wave of the OB speaker was interacting with the sub at some frequency, causing cancellation perhaps?
I mention this because it may very well be that some sub/ speaker interactions will not be affected by the switching of phase- sealed box speakers maybe, front ported, etc., on and on. Maybe rear ports with subs behind the speaker are affected differently.
This could explain why some people hear no difference when flipping phase- I have a separate home theater system, and for the life of me I never heard a difference when doing so. In this 2.1 channel system, it is readily apparent to me.
As I write, I also realize that the 2.1 system uses a sealed sub- the HT subs were ported. Maybe porting changes things too. Just more to consider...