I believe the Bag End approach is to not have a 'real' crossover and the group delay associated with it, and operating it below resonance as Bag End have done avoids the group delay completely. It is essentially two 6db/oct lowpass filters set around 8Hz-20Hz. Linkwitz did a similar approach on his site by raising the crossover frequency and lowering the slope to only 2nd order, because he had group delay problems with the sound.