Put simply the gain control is just a way of controlling how loud the amplifier is when controlled from a seperate source. If the gains are set higher, then the amplifier will be louder at a lower volume through the source you are using to control it. Turning the gain down lower will require the volume control to be turned up higher at the other end.
It has nothing to do with power output from the amplifier, the amp puts out the same amount of power regardless of where the gain is set at.
If you have the gain set so that you can reach a higher level of volume on the head unit, then you get better volume adjustment as the increase is made in smaller incriments allowing you to be more precise.
Just set the volume at a reasonably high level on your head unit ( if it goes form 0-40 then say about 30-35) Begin with the gain at it's minuimum settings. Very slowly adjust the gain, until you either are aware of the subwoofers limits, there is any audible distortion.
If you have a seperate amplifier that runs your in-car speakers, then you need to repeat the same process. Always set your gains at the same volume on the head unit so that your previous adjustments for any amp remain the same.
