Active Crossover Linkwitz-Riley 12dB for BiAmping

I did like you said. Put 2 Allpass in series.

Now how it looks.
Red = Lowpass, Blue = Highpass before allpass
Green = Highpass after allpass, f=2510 Hz
Libkwitz-Riley AC Analysis_03 2xDelay.jpg
 
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I guess the proof would be some kind of test against a known control. The difficulty vs dsp I suppose is how does the user calculate/fiddle/set the delay depending on the drivers and baffle layout/cabinet used, per design,
If you see delay as simply phase in the context of a band of frequencies, should it matter how much actual delay you use? In a sense, an all-pass filter as a tool is just an opportunity to alter phase free of added response variations. Your goal at the end of the day is only to have the needed relative phase and response relationship between the two ways at each frequency around the cross.

Alternately, sometimes you have some wiggle room with regard to the relative response relationship over parts of the crossover region, and can afford to manipulate a response slope to create the effect of delay. You can then use global EQ to fix it without harming the relationship, especially given that many are already planning to EQ globally regardless.
 
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When the four resistors are 6k34
the crossover frequency is 2510 Hz. See image.
By changing any frequency can be achieved.

View attachment 1414293
Selecting the crossover frequency depends on individual drivers, especially the beaming of midbass. Midbass will start narrowing above certain frequency and may show some breakups. You naturally want to supress breakups by selecting lower crossover. This however presents challenge to tweeter. Smooth of axis is the key. Here is more about beaming and of axis response.
http://www.troelsgravesen.dk/Beaming.htm
 
I used 2 all pass filters in series, it produced better results. For me. I had to delay about 22uS which doesn't seem like much but it change things.

I did it the cheap way; I adjusted the delay until the best null showed on the measurement between the T and M.