I'm experimenting with not just matching/optimizing via a fixed distance delay, but using IIR all-pass filters to mate the group delay of different driver alignments.
For reference, I have four sealed subs (4 cubic footish) and two ported (12 cubic footish, ~16 hz tune) in the theater and thought this might be interesting. I started in winisd and played with all-pass parameters on the sealed sim until the system group delay matched with the ported sim. It's possible to get very close with just guess/checking (though it would be nice to write an algorithm to solve for this, particularly for active crossover design of multi-way speakers). I then put those biquad coefficients into a minidsp and ran before/after REW sweeps of the subs separately and together in-room.
Theory matched practice pretty closely. The trouble is that, while the phase plots in REW when measured at the seat look fairly close (and much tighter than without the all-pass), when I turn on all subs and run a sweep, the frequency response is markedly worse (bumpy with deep funky null). I'm wondering if part of the problem is much of the phase shaping happens say <30 hz, and there it's more about room modes than first-arrival. Alternately, I've got one sealed sub near-field right up against the seat with the rest dispersed around the room at roughly 10-15 feet distance, and that might be further confusing things. As it is, everything sums ok, but I've always thought it could be better.