Critical subwoofer tip

I've got 2 DIY 10" subs.  They are made out of nice birch plywood.  The cabinets do not vibrate. They are ported.  They are, heaven forbid, wired in stereo.  They are crossed over at 100Hz currently.  They will play 105dB @ 30Hz. All my crossover has is level for highs and lows to compensate for gain from the amps.  They have sat in the same place for 20 years.  I always thought I had good bass.  Then, when I retired I started taking this seriously and put bass traps and other sound treatments in my room.   Now my bass is really nice.  It was actually boomy before when I thought that is what it should sound like.  I can walk around the room and the sound is the same.  I think room treatment is much more important than these expensive subs with all these controls.  The only reason a sub should cost more is better cabinet construction and nice drivers.  All this other stuff leads to chasing your tail.  What is much more frustrating to me is how recordings are made and bass levels vary.  My true test is to find a recording where they hit a timpani as hard as they can and it still sounds right.  I've got that.

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....