I have been waiting to see if someone with technical knowledge would respond to your post... I am not a tech but I recently biased my DSL 50. Like you I read that the knobs should be set to zero (not sure how important that is but this is the way I did it). I set all my knobs to zero and then biased to 42mv using the video on eurotubes as a guide:
euro-m
The above procedure seemed to work well for me.
Dennis
