I would think that the three basic types of crossovers, passive, active and digital, would handle the division of frequencies in different ways. Not being a crossover designer, I may be speaking out of hand, but I’ll offer my thoughts in simplistic fashion for discussion.

If a single stream of data is divided by some means into two or more streams based on frequency, and then each stream put through the same type of D/A conversion, the resulting outputs, even if different in phase from the original input, would, in a decent design, still be properly in phase with each other.

Similarly with an active analog crossover, where the division of frequencies are usually made by complimentary resistor-capacitor networks in association with op-amps, there is a phase shift, but my understanding is that all frequencies shift in the same direction, and at the point of crossover, by the same amount. The points of crossover are the most important, where some small portion of the signal will be sent in two directions and would hopefully be reproduced in phase with each other.

Passive crossovers, now there’s a different animal. At each crossover point, both a capacitor and a coil are usually used in the division of frequencies, the phase shift being in opposite directions. Depending on the complexity or simplicity of the crossover, if the output to one driver leaves the crossover in near opposite phase to another, one driver may be wired to the crossover in opposing phase to another driver in order to help restore the proper phase relationship of the moving drivers.

In a pre/pro or other device dealing with bass management, it would seem to me that three reasons exist for not giving the consumer phase adjustment controls. One, the implementation is unnecessary. Two, the implementation is complex and/or would add cost deemed by designers/manufacturers not to be an advantage when marketing a device to consumers, unless that device is meant expressly to deal with the phase issue. Three, giving most consumers more to adjust might lead to increased frustration for some users who would rather have things as simple as possible.

Most powered sub manufacturers give the consumer the option to switch the phase relationship by 180º, and the same option exists for passive subs by reversing the speaker leads. Some powered subs have a variable phase adjustment. If the method of dividing the frequencies is consistent for all frequencies, a one time adjustment of a subwoofer should be all that is necessary. I don’t that kind of consistency is too difficult to achieve.