You will probably have to find someone who has heard this static before. One thought is to record the static on your phone and upload the audio to a free online storage service like Google Drive, check the box to enable everyone to download.
So far you have tried all the easy diagnosis options. The noise doesn't appear to come from a source or interconnects, it happens with multiple amps so it can't be dirty tube pins. Is there a 5G transmitter close by? Lots of facts mixed with fiction regarding 5G so if your avatar starts wearing a tin foil hat we will know you went down that rabbit hole. Does your noise sound like this?
https://biome-living.com/sound-of-5g/
I suppose the next step (after the cheater plug) is to check for noise on the main line either at the panel or on the pole. I have no experience with this type of noise but other AC members have.
Mains noise suppression - seeking for advice
https://www.audiocircle.com/index.php?topic=162241.msg1727433#msg1727433
"Search" for "dirty" in AC, check "The Lab", "The Path of Least Resistance", and "Power Conditioning"
What electronics have you tried? All we know is they are Class A/B and Class D. I recently heard a Buchardt I150 Class D integrated driving a pair of Fritz Carbon 7 Speakers at Fritz's house and it was very nice, it didn't sound like class D at all, I would have never guessed. You wrote Class D was silent, might be the way to go. Give Fritz a call, his speakers and the Buchardt is a killer combination.
I will work on getting static sound uploaded and share it here. No 5G around. The creator of my amp is going to bring this device
but only some time next year. He doesn't recommend removing ground although I guess I can do a test.
The amps I tried are Eon Art Quark (keeper), Parasound A23 and Cary Audio 200.2. Class D is Nilai500. Actually Quark is class A + D amp but it doesn't have switching PSU.
