Krell PAM-5 gain adjust

I have a Krell PAM-5 pre-amp where the output has a RMS difference of 0,2 volt at my favourite listening sound level.
I think it should be possible to adjust the gain so that both output channels have the same RMS output level but I am not sure how. I can do it by trail and error but I first want to try it this way.
Can somebody help me with this?