For what it's worth, my 755 has always been a bit sensitive to line noise as well. When I lived in an older home with fairly ancient wiring, the only thing that worked was to put the entire system on a very fat UPS (APC Matrix 5000) that I had "laying around" when I decommissioned it from the office datacenter.

I live in a much newer building now with more current wiring and the 755 is quiet without the UPS.

Best,