Andrew (Drewan77), thanks again for your always excellent inputs. A couple of minor clarifications to your post just above, if I may:
Sound waves travel at different wavelengths/speeds from the lowest/slowest to the highest/fastest frequencies so what you are aligning to is a subset of all.
Although the velocity of a sound wave in air does indeed vary as a function of frequency, as I understand it the amount of that variation is small enough to be negligible for practical purposes. See the graph near the lower right corner of
this paper, where it can be seen that even under the worst case condition (0% humidity) a frequency of 10 Hz is less than 0.03% slower than a frequency of 20 kHz. At a listening distance of 3 meters, that would result in a propagation delay difference of less than 0.003 milliseconds between those two extremely different frequencies.
With a single full range speaker containing passive crossovers, or a 2-way, 3-way measured accurately at once (ie without requiring subs), then an appropriate DEQX processor will do everything for the user and it automatically becomes time and phase coherent.
But of course only to within a degree of accuracy and over a range of frequencies that are constrained by the duration of the correction window and by reflections that may be captured within that window.
Thanks again, though, for another excellent post. Best regards,
-- Al