The amp uses KT120s
Per the amp’s manual:
Using the bias tool, slowly turn the screw on the potentiometer that is adjacent
to the switch until the bias meter reads ~ 35 miliamps (mA). Turn the screw
clockwise to lower the bias and counterclockwise to raise the bias. Note that
there are two ends on the bias tool. Using the end with the recessed
screwdriver will greatly facilitate this operation.
9) Flip the switch back into the “run” position thus disengaging the tube from the
meter circuit.
10) Repeat the above steps for each of the tubes in both channels.
As it is a quick operation, the tube bias should be checked on a regular basis. Note
that the bias will change slightly on a day to day basis as the wall voltage
fluctuates. Constant biasing will wear out the bias potentiometers prematurely so
do not adjust the bias unless it is more than 4-5 mA from the correct setting.
The Op didn’t state how many approximate hours he has on the tubes, but unless they are all nearing end of "expected life" , there’s no reason to replace all since each is individually biased. That said, the bigger immediate issue for the Op based upon a google search is that new KT120’s appear to be out of stock at a number of vendors.