Turn the bass boost all the way down unless you notice a dip in the frequency response around the boost frequency. Then turn it up enough to level the sound. Most people can't do this by ear.
Set the gain to match the level of the other speakers. The system should play as one harmonious unit. That is unless all you care about is the bass in which case set it wherever you want.
The gain is solely used for matching the input voltage from your head unit. You are really supposed to set it with a DMM (digital multimeter), but read the kicker manual, they have an explanation there.
