After you break it in a little under very moderate gain settings, Turn the gain to lowest setting. Put in a CD you know well, and crank up the voume to about 70%-80% of max. Depending on your midrange speaker amplification, it can be loud. The idea is to have it near the upper range of volume without any clipping or distortion. You'll want a little extra room for when your favorite song comes on, etc.
Anyway. slowly inch the gain up until you hear distortion or muddy bass. Turn it back a few degrees. Now check crossovers, adjust any signal processing options, and recheck the gain. Now you can lower the volume on the head and your gain should be set. Recheck it in a couple of weeks, and then maybe a month after that.
Long term, you'll want to match amplifiers (this is why it pays to stick with one brand) so they have matched volume ramp up based on input voltage. Basically they volume of midrange and sub should match no matter what voltage input/volume on the head unit. Different brands may have different tolerances leading to overpowering bass at low volumes, but when turned up the highs become too over powering. Anyway, something to think about as it looks like you are just running 1 amp currently.