i use 0db tone when i tune mine. i know which songs will clip and make my sub stink. and i know which one is quieter and increase the bass level a little bit on the head unit. remember, listening to song off your AUX can clip easier than using a CD.
for example:
i have a apple tv 1g that i use as my main source of songs, videos, pandora, etc to bump. i use it off the AUX. when tuning my amp off the deck, as everyone knows i use the CD for the test tone. i tune mine 59/62 off the cd. BUT when i listen to music off my apple tv, I know my max is 53 because anything above that volume will start to clip and get stinky.
just my 2 cents