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).
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).