Thanks for all the responses!
noromance: Wifi routers etc can't affect the system if their AC circuits are turned off, or can they? I tried turning all breakers off except the sound system, and the hum persists.
russ693: I think I tried reversing plug polarity when I tested cheater plugs, but will try again to be sure. I have disconnected everything except the preamp and amps and it still hums. I've swapped interconnects. No difference.
petg60: Regarding checking with multimeter, would you clarify exactly what should be connected and what I should measure? I'm a bit ignorant of electrical stuff so just want to make sure.
millercarbon: Hilarious... and agree with everything you said. At year end I said to myself "Hey why don't I frustrate the cr%p out of myself again and try to solve the hum again?" Yes, it does not seem to be the preamp per se, but the interaction with it. And the only things it can be interacting with are the power supply or my amps... assuming this is not coming from the air in terms of RF or something.
slaw: As I noted, unfortunately I can't, at this point, remember when the darned hum began. I realize that would be very helpful.
erik_squires: Never thought to check if one amp only relieves the problem! I will check.
ozzy: Interconnects are all RCA. I have not run the sources straight to the amps but I have replaced the preamp with a preamp section of a cheap receiver, and the hum goes away.
noromance: Wifi routers etc can't affect the system if their AC circuits are turned off, or can they? I tried turning all breakers off except the sound system, and the hum persists.
russ693: I think I tried reversing plug polarity when I tested cheater plugs, but will try again to be sure. I have disconnected everything except the preamp and amps and it still hums. I've swapped interconnects. No difference.
petg60: Regarding checking with multimeter, would you clarify exactly what should be connected and what I should measure? I'm a bit ignorant of electrical stuff so just want to make sure.
millercarbon: Hilarious... and agree with everything you said. At year end I said to myself "Hey why don't I frustrate the cr%p out of myself again and try to solve the hum again?" Yes, it does not seem to be the preamp per se, but the interaction with it. And the only things it can be interacting with are the power supply or my amps... assuming this is not coming from the air in terms of RF or something.
slaw: As I noted, unfortunately I can't, at this point, remember when the darned hum began. I realize that would be very helpful.
erik_squires: Never thought to check if one amp only relieves the problem! I will check.
ozzy: Interconnects are all RCA. I have not run the sources straight to the amps but I have replaced the preamp with a preamp section of a cheap receiver, and the hum goes away.