I would rule out what you mentioned below before putting my amp in the mail. Someone else chime in as I also have zero technical knowledge. But all my troubles have always been due to tubes being bad/going bad.
To all- I will try swapping out 300bs but it's also hard for me to believe that all 4 were identically bad, resulting in identical hum in both channels. Again, if I'm off base on that, please let me know. As I've said, I have ZERO technical knowledge of how these things operate.