buy a DMM and adjust with the subs unplugged //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
if it's loud at 1/5 gain, maybe that's as high as you need to go.
just to make sure you know the higher number next to the gain knob means quiter (less sensitive)