Personally I don't buy the voltage fluctuation thing. Voltage would have to vary quite a bit to affect the amps power supply. Voltage and frequency are very tightly controlled by the power company. When the hum starts in the evening do you notice a change in the intensity of your lighting or anything like that. Could the power cord to the 7700 have been damaged at all. Can you try another cord. You may be able to borrow one from the guy that worked on your amp.
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Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner