Yeah man, just download some test tones and use the DMM tutorials... you can pretty much ignore the "Back it off once you hear distortion" crap. As for your headunit volume... different ones clip at different levels. Honestly, 3/4 of the way up actually is somewhat useful in this scenario... you're saying your h/u has 2 volt pre-outs, so I'm assuming it's a lower level one like mine. Mine's a Pioneer deh-3200ub, and I never go above 53...