My tech told me how to do this over the phone and I wanted to share. With the cabinet hooked up,the amp on and warmed up,stand-by off ready to go,volume cranked up , NO cord plugged into the input............, Measure mV from K1 to ground. The schematic states .072 volts, so use the bias pot (VR1) to set it to .072 volts. Then measure from K2 to ground and adjust VR2 until you see .072 volts. Now go back and see if K1 has drifted. If so ,readjust it back to .072. Go back and forth checking and rechecking. You won't believe the difference in clearity if it was off much.