normally its done this way for the speakers and sub separately. set the speakers and once those are done, tune the sub
Amp gains at minimum and head unit at ? volume. Play some well recorded music that you?re familiar with. Turn the gain on your amp up until the music doesn't sound quite right. Be it clipping from the amp or distortion from the speakers. That?s it, your done, leave it there.
Down the road if you find yourself turning the volume on your head unit to almost max on a lot of music, turn the gains up higher. If you see that your going less than half volume before it doesn't sound quite right, turn your gains down a bit.