here is a picture show how the amp6 module work with a special edition DN4001.
you can see the left is for most headphone amp with a 4001 hybird headphones. but question is the sound is not nature cause the passive filter can seperate the bass with treble/middle very well,
with the amp6, it can drive the bass and treble/middle directly , and the active filter has far more better performance than the pass filter. ( low pass, band pass, high pass.....).
there are quite some expensive speakers with this design. of course, more active speakers only drive by 2 channel amp .