As Siegfried Linkwitz pointed out in Crossover Topology Issues:
“An optimum crossover filter topology should be the starting point when the outputs from multiple drivers with different frequency responses and sensitivities need to be combined for a flat acoustic response”.

An ideal phase relationship between the channels leads to perfect summing, if the drivers’ acoustical responses are sufficiently linearized.

Find a few more thoughts on how to optimize filter topology in a 4-way speaker:  Filter Topology 4