They claim that if you buy the Fender labeled 6l6 tubes that they do not have to be re-biased.
I have never changed the tubes in mine, so I cannot verify that statement.
Lefty
I believe I read the Fender brand tubes were color coded to indicate hardness/softness. I like mine just the way it is. I used it for 6 string in the last band and it worked out really well. Not a backbreaker either.
Lefty