Mcintosh. Google it. They will refer you to an authorised dealer, closest to you. north or south pole. :-)
No there is no adjustment, for the bias. It is fixed.
AGAIN, that 12AX7 or 12AZ7 in the right front corner, can cause a problem...
Did it do it before the valve swap? If not THINK?
Why would it need to go to the shop AFTER a power valve change?
What did you do, or it do... OR is there anything wrong to begin with..
You have two of these things, I own one.. of the GG models and one of the older 62 models.
Red plating will burn the new valves right up.. If there isn’t a problem with the valves to begin with. There will be soon..
Get the Old valves and check it out.. FIRST don’t keep playing it...
Your not bi amping and one is bass duty, is it?
That could get the plates a little brighter, I suppose, not much though..I don't know, have to think on it..
Regards