Low level buzzing coming from tweeters.

Usually the AC power comes from two halves of the incoming 240V from outside the house. If the amps are on one half and the rest of the system on the other, sometimes you can get hum if the amplifier's grounding isn't really worked out all that well.

So the extension cord may be going to an outlet on the same side of the AC power as the preamp's power is.

If OTOH the preamp and amp share the same outlet than something else is at play.

The system, including the amps, are on the same (right) side of the electrical panel. Should I assume that means they are on the same side of the AC power?

The amps are on a dedicated 30 amp breaker, the rest of the system on a dedicated 20 amp. There is an outdoor imbedded ground for, I believe, the entire system.

This excessive hum arose well after the dedicated lines, panel, and ground were installed. It probably began within the past year, maybe during the summer. I wasn’t paying much attention as I was in constant shoulder pain, then surgery, so I can’t really identify exactly when I first heard it or anything that might have occurred which could have precipitated it. 

The amps are on a dedicated 30 amp breaker, the rest of the system on a dedicated 20 amp. There is an outdoor imbedded ground for, I believe, the entire system.

Is this a stake that someone put outside somewhere to make a ground?

No, if the system is all on one side of the breaker box with one on top of each other then that is causing the hum. The breakers are staggered.
https://images.search.yahoo.com/search/images?p=breaker+box+inside+pictures&fr=yfp-t&imgurl=...
https://i.pinimg.com/originals/b1/04/95/b10495877e448c7626e2b9046f295001.jpg

ozzy

@atmasphere 
Yes, it is a pole, about 15-20’, driven into the ground with ground wires fused to it.

@ozzy 
Yes, the dual 30 amp breaker is near the top of the right side of the box and the 20 amp breaker is also on the right side near the bottom. There are five “blanks” and two more breakers between them.

I’m trying to contact the electrician who installed the panel and the outside ground.

I appreciate all the input!