After reading through the thread I am unsure if you tried this. After 20 years in the same house my speakers developed a hum one day this year. I tried unplugging everything one at a time including the cable box to no avail. Still had a hum. At the suggestion of the amplifier manufacturer I disconnected the COAX from the wall to the cable box. No hum. Installed a COAX inline isolation transformer and no hum (or maybe 99% less hum). I was misled thinking unplugging the cable box was the same as disconnecting the cable box COAX in. FWIW.
Jim S.