I finally got around to trying it with my new Beyerdynamic T1 headphones, and I can report pretty much the same effects as with the HD650. The sound moves a little bit forward and seems more natural and effortless to listen to. There is also a slight filling in of the tone. Seems that maybe it is boosting the mid -range since that is the only frequencies I hear when I turn down the main headphones and only listen to the Koss on-ears that are hanging from a headband and sitting outside the bigger over-ear Byerdynamics. So maybe along with a completely natural crossfeed, I am also effecting a natural EQ like boost of the mid-range. This is actually ideal with the T1 headphones as their treble is a bit bright, and so boosting some mid-range fills in the tone.
Again the effect is very subtle. I added a in-line volume control to the cord to the second pair of headphones so I can easily turn them on and off. Even though the effect is subtle and hard to pin down, I definitely sense a more natural soundstage that is like well placed speakers. There is a greater degree to which I can forget I am wearing headphones and just listen to the music.