Aha. Very interesting! So that explains it.

For the record, what the stereo separation does is this:

Let's say you've got a Beatles track or some other similar track where the voice is panned hard left and the instruments are panned hard right.

If you listened to that with a pair of regular speakers, you'd get an interesting stereo sensation, because both ears would get a bit of each speaker. If you listened to it with headphones, then it would be jarring because each ear is totally discrete.

What the stereo separation feature does is to mix the left and right channels back a bit towards center, so that if you were listening with headphones it would sound more like regular speakers would.

Also, if I recall correctly, it slightly delays the leftchannel mix as it goes into the right ear, and vice-versa, so that it simulates the extra time it would have taken for the sound from the left speaker to reach your right ear.

(Please note that this feature is available on some very expensive audiophile headphone amplifiers, for the same reasons.)

So, I would guess that doing this might cause certain frequencies to be enhanced more than they would otherwise be with a regular stereo field. The bass I would think would be most noticeably different because of the lack of phase cancellation.

Very interesting.