@ustk
Yep! A lot of people get caught out with that.
Because it actually sounds pretty close to correct, if you use an allpass on the high band in the way that Hise's examples do.
But rather than getting a notch eaten out of your spectrum, this will cause a little bump instead.
a3da661a-6e5a-4dca-87e1-c7928cd9edd8-image.png
Which is subtle but it still exists. It's most noticeable when you put two crossovers near the same frequency, you get a dB or more bump in the spectrum.