Low level buzzing coming from tweeters.

At a friend’s suggestion, I ran a long extension cord from each amp separately to an outlet at the other end of the house. There was now a barely audible hum that could not be heard about 2 1/2 feet from the tweeters. My listening position is 12 feet from the speakers and the buzz, which I now better describe as a loud hum, was loud enough to be annoying . . . and aggravating.

This makes more sense to me as I presumed the odds of both amps having the same exact problem to be unlikely.

Next stop . . . an electrician.

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