How do I tune both my Amp and my Line Out Converter?

I am confused on how you tune an amp and line out converter at the same time. I have an LP7-2 LOC, and R1100M Amp and they both have gain knobs on it. I understand that for each, if tuning by ear, you need to put your radio at 3/4ths volume, play a 40hz test tone, and then turn the gain knobs up until the subwoofer starts audibly distorting, but how do you do it both? The only thing I can think of is to just leave the LOC at 100% gain and then turn the amp up until it starts to distort and then leave it right before it distorts, but is that really the right way?