If you have both 10KΩ trimpots installed on each amp board, then you use them to adjust the bias and offset.
First, make sure that the DC servo opamps are unplugged, then power-up the amp and let it warm up. Set the volume control to minimum setting (if equipped). If not, then short the left and right inputs to ground with a 1KΩ resistor.
Measure the voltage across any of the 20Ω output resistors. For the proper amount of bias, you should get 0.75V (which corresponds to 75mA of current on each output transistor branch). You should turn each of the trimpots to increase or decrease the voltage. If the voltage is too high and you can't reach 0.75V even at the minimum trimpot position, then you should replace the 500Ω resistors that are in parallel with each of the trimpots to 620Ω or 680Ω and re-test. Once you get about 0.75V on all the 20Ω output resistors, check the output DC offset. Turn one of the two trimpots to change the offset toward 0mV, a little at a time, and simultaneously turn the other trimpot in the opposite direction. Re-check the voltage across the 20Ω resistors, make adjustments as necessary. Repeat this until you get less than 1mV of DC offset, and still have the correct voltage across all those 20Ω resistors.
Do the other channel the same way.
Wait 15 minutes for all voltages to settle, re-check and readjust slightly if necessary. Once you're done, power off and let all capacitors drain down. Install the DC servo opamps and power up again. Check the DC offset to make sure all is well. Remove the 1KΩ shorting resistors at the input if you had put them on.
Hope this helps.