Cleaning up of the mid range is exactly what I saw ( heard

).....direct positive affect on the mids. By Bi wiring the AMPs Dampening is directly in between the Crossover at the Woofer/MidRange point.....
From an Ohm's/Foot point of view for the wire....Bi-wire now favors the AMP dampening, rather then the Woofer being closer to Midrange driver then the amp in a normal set up....
In a normal wiring plant, my take is the Midrange driver is acting as a damper for the Woofer's Microphone EMF in frequency range of the crossover overlap of the Woofer/Midrange. And that is the source of distortion that is cleaned up once you move to the bi-wire.
jk