Sat(still had Charles on my mind from a previous reply for some reason), no I can't even hazard a guess as to why the situation would be improved at the lower impedance setting(if after switching back-and-forth that still appears to be the case). The suggestion about the pre-amp output wouldn't appear to be part of the explanation either(the lower impedance setting affects only the internal amplifier in the 663 and has nothing to do with the pre-amp section). The mystery remains.

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Enjoy the music, not the equipment.