401s gain settings

Turn your gain all the way down on your amp. Now turn on your head-unit and put in a cd you're familiar with and turn your radio up to about 75 to 80 percent. Now, turn your gain up on your amp until you hear your sub starting to distort(or the maximum level you want it to play at). Once you hear it start to distort turn the gain down to just before the point where it starts to distort.

eh mine never starts to distort

Then do the same procedure as above and set the gain to loudest you want your sub to play. This isn't a law though. If you adjust it this way and find that there isn't enough bass at low levels then adjust to however you like it(just avoid turning it further then 3/4 of the way up or abusing the bass boost control if you have one on your amp).