Sounds like a bad tube to me, too, especially if the hum is in one channel only. It might just be a tube that needs to be reseated in the socket.
Whatever you do, use a step by step method. Consider swapping tubes from left to right, one pair at a time, and listen for changes.
Whatever you do, use a step by step method. Consider swapping tubes from left to right, one pair at a time, and listen for changes.

