Speaker hum.

Thanks for all the suggestions, everyone, I’ll give them a try. Yes, the units are indeed old and servicing history is unknown-a common risk when buying such equipment. I would guess these are north of 20 years in age given their introduction year. Anyway, I only noticed the noise when I leaned over to pick up some debris off the floor while just inches from the speaker. Otherwise, I’m not certain I would have noticed it at all. Thanks again.

Have you ever had the units serviced? The models you have could be 20 or more years old. It is very common that the power supply capacitors need to be replaced at that age. 

Power supply capacitors, especially the big ones on a power amp, are like tires and brakes on a car. At some point they wear out and need to be replaced. If the capacitors are shot, nothing other than replacement will fix the problem.

Put a ground cheater on the amp to "float" the ground and the hum will go away. You can also break the ground lug off of a spare power cord.

OP,

Sorry you cannot put in direct lines… two is standard. One for components and one for amp. It would improve sound quality and take care of ground loops.

Before I got my second direct line, I just had to plug everything into a single outlet.

There are some fixes that may help. You are using two different circuits for your system… if you replace all the duplex receptacles on each line this may be what is causing the differential resistance between the two. You’ll have to do a bit of exploring to find them all. 
 

I am not sure this really does you any good. But make a heavy duty extension cord… I used 10 gauge and plug the second half into a nearby outlet that is on the same circuit as the rest of you system. That is basically does the same thing as plugging in to the same outlet. 

I gather you mean independent AC outlets for each component? Unfortunately,  not really an option.

Direct lines for your equipment… better than sharing.