Per Schubert, there is a distance at which a 1st order crossover theoretically maintains phase coherence - it will vary with the physical properties of the cabinet/drivers - but it's hard to otherwise generalize.
As to extreme near field use, single driver and/or concentric driver (see KEF uniQ) systems are one way to dance around the issue.
Marty
As to extreme near field use, single driver and/or concentric driver (see KEF uniQ) systems are one way to dance around the issue.
Marty