I am guessing here since I don't have my 57SL in front of me but since you set cross-fader in the middle and music dropping in gain, it could be related to the -3dB dip that the cross-fader curve has in the middle when set to the medium speed.
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To confirm, does the volume drop in the middle of the crossfader path and then come back at the end of the crossfader travel? If so then it is likely (assuming all else is working) the -3dB dip. Try moving the cross-fader while the curve control is on max fast setting. Does it not dip then? It shouldn't.
If you feel strongly about the -3dB dip, please post in that thread above. :-)