imo people are overcomplicating.
play around with rough delay jumps until you get excess group delay close to zero (use dirac impulse for audible pre-delays).
from there play around with finer delay steps until you get the flattest in the crossover region
This is even more obscure to me than the OP. Excess group delay of which measurement (sub or main or both?) I don't understand the comment about Dirac impulses. Actually play them through the speakers?