Actually the Mid Monos don't even need a meter . To bias you just turn each tubes bias pot fully counter clockwise . Old Mid Monos have one potentiometer per amp to bias and for those matched tube pairs for each amp are recommended. The newest version has a bias pot for each tube which is better.
So with all pots counter-clockwise, turn the amps on and let warm up for a few minutes
I recommend doing one amp at a time.
After it has been on for a few minutes, press the bias test button, there are two for new units. Press and hold the button as you slowly turn the pot clockwise.....slowly. the LED will start to glow. Just when it starts to glow, back off a little until it goes out. Let the amp warm up for 15 , checking the single or both LEDs if so equipped to make sure it doesnt glow bright RED. If it does back off counter clockwise a little.
After the amp has been on for at least 15 or 20 min check bias and adj clockwise until the LED just starts to glow red. REMEMBER TO PRESS THE LED CHECK BUTTON AND HOLD WHILE ADJUSTING.....
The goal is not to bias them hot for a long life. If the LED barely glows the amp is in the correct range.