Ohhh. Thanks for this answer. I'll look at this more closely. So you are talking about something like this? https://www.pooraudiophile.com/2015/03/how-to-fix-dc-offset-and-transformer.html
I'll post a reply later with more info. Thanks!
Here is at least all you wanted to know about causes and effects of DC on powerlines and how to block it.
At least one company, Bryston, includes DC blocking in at least some of their power amplifiers, for example the 4BSST2. The schematic is online.