Low level buzzing coming from tweeters.

@cleeds 
I feel somewhat relieved to hear this, as I have had confidence in my electrician who has done some extensive work for me, including upgrading my electric service, installing a new panel with a built-in transfer switch for my generator along with the outdoor plug for it and ground, as well as the separate ground for the audio system.

As I am not knowledgeable in these matters, I must rely on others for their expertise. The electrician is tentatively scheduled for Monday afternoon.

1. The first thing I think of is a ground loop between cable TV and house ground.
Disconnect cable TV from the entire house, and re-test.
(the CATV ground should be entirely isolated from the house ground)
If noise goes away when CATV is disconnected:
In order to cure this, use a cable TV isolation transformer which is designed to stop audio buzz by isolating the cable ground from the house ground.
This type isolation transformer can be purchased ready made -or - you can make one by connecting 2X 75 ohm to 300 ohm antenna transformers, you connect the 300 ohm ends wire to wire. Then you connect your 75 ohm cable (in/out) to the coax ends of the transformers.
This will make an effective isolation transformer for about $10...You can make these out of Radio Shack antenna transformers.

2. Second thing that comes to mind is an ordinary ground loop.
Allow the preamp to use a 3 prong grounded plug. Then lift all other grounds (at the mains power plug) using adapters.
These power plug adapters (2 to 3 prong) are available at the hardware store.
You need 1 adapter for every piece of equipment that has a 3 prong plug.
Including: Lift TV, Lift sub woofer amp, etc...until all the grounds are lifted at the power plug, except 1.
This type of loop occurs when there are repeat grounds (more than 1 ground). The equipment is already grounded together using the audio cables...and the video cable.
Then, you ground everything to the mains power (3 prong plugs) and this makes the loop.
By lifting some or all of the grounds...you can stop this loop.

I’ve tried both of these. I don’t have cable TV. I’ve long had only the pre-amp with a three-prong plug.

I believe the issue may be deeper in the electrical. I had an electrician out who does installs for a high-end audio shop and he couldn’t find anything.

Could the power company’s electric feed be the source? The other night I was not getting the hum/buzz for the first time in a long while. This confounds me even more . . . 

Buy an receptacle tester at the hardware store.
This is about $10...
This has 3 lights, that tells you if the ground is good, and if the power is wired correctly.

Yes there could be something wrong with the utility.
This type problem is analyzed with test equipment, that detects noise or harmonics in the incoming power. (Fluke power analyzer)
A specialist will have this type equipment available to test with.
But you can not usually rely on the utility to detect or fix a problem.