I can never remember what the acronym stands for, but it is an extended/FR crossed over to a helper woofer <300Hz.
Phase becomes much less of an issue. At 300 Hz a half-wavelength is close to 23 inches. You can organize the drivers to all fall within a sphere that size, making them essentially co-located.
Use some good mid-bass capable of >1 kHz and you can get away with a 1st order XO -- if passive, a good place for a series XO.
An example:
dave