I thought you were supposed to supply a set frequency/sine wave to the amplifier when you're measuring signal strength? Then you set the gain according to that? i.e. volume knob at ~80% with a 100hz sine wave. Use your DMM and the rest is math and gain knob?

EDIT: http://www.jlaudio.com/header/Suppor...Setting/287546

I know fatboy gave a link but sometimes different pictures make things click for different people