Received my plate choke replacement today (thanks, Eileen!), swapped out the defective one and the amp, at first, had no sound in the right channel. I checked voltages, which were slightly low around 6, 21, 22 (51.7V compared to 56.something on the other side), but all else well, so I plugged it back in and …
… it works! And it sounds great! And there is no more noise!!
Thanks so much for helping me get to the bottom of this!