All good tactics for finding and eliminating the problem. You should also look at RF issues: Bluebird Picking Up Radio?
Remember Occam's Razor, the easiest and most obvious problem is usually the correct one. Examine what changes you made to your system and go from there. Unless your power supplier (typically a government office) changed something on their end it HAS to be something that you changed on your end.
Now having said that you need to know that you can also get a buzz when tubes, caps and other electrolytics fail. Go backwards from now until then and if that presents no solution start with oldest gear to newest.
Of course we all realize that electricity is still a theoretical energy force....Its a lot like predicting which way the water will run without all the info.