Sajjad, just want to make sure you're on the correct board. AWS-080 is the Switch Assembly board which is accessible from the bottom of the amplifier. There should be a black wire on the pin that routes through to the top side of the chassis near the power switch. Pin 21 should already be grounded to the chassis but I found that there is roughly 0.5 ohms of resistace to ground which is providing an opening for hum to be introduced.
Is it possible that you are looking at pin 21 on the Control Amplifier board which is it's right channel signal output?