If it is one channel, you can isolate the offending tube (if it is the tube) by swapping left and right tubes to see if the problem follows a particular tube.
He did this and mentioned it in the thread :
I did this by swapping the left and right output tubes and the noise moved with the tube.
I am running used tubes with a reasonably low but unknown amount of hours,
You have done the process of elimination , so guess its time to buy some different tubes.