Unity Audio used a capacitative network in their speaker crossovers to actually time align the various frequencies that propogated from each individual driver. Apparently, in a driver's bandwidth, the lowest frequencies arise from the periphery of the cone and the higher frequencies are emitted toward the center. So, in a cone configuration, there is time domain smearing from an individual driver. On top of this, the alignment of multiple drivers is the secondary (and possibly overlapping) source of misalignment. Imagine that a single frequency near the crossover frequency is played by both the midrange and the woofer (for example) so that there is smearing of the same note from two drivers.
Time domain smearing can be a complex problem, indeed.
While I don't know the exact science or implementation behind the crossover design in the Unity Audio PARMs I had, they sure sounded coherent. They also received rave reviews at CES, etc. Unity Audio founder Bob Grost, who now heads Cerious Technologies, was an engineer for sound cancellation in Corvettes for GM, if I'm correct. He's a good guy, and I hope he comes out with some statement projects for our community to examine.
Lee