I'm pretty sure the whole idea of tuning your system and setting your "gains" is so you can max out your volume on your head unit without distortion. (REGARDLESS OF WHAT BRAND OF HEAD UNIT, THE SAME IDEA SHOULD REMAIN THE SAME ON ALL AND ANY, BUT EACH SETTING WILL CLEARLY BE DIFFERENT FOR VARIOUS SETUPS)
Practically speaking, you want to tune your HU at 75% volume (ie. if your MAX volume on your HU is 60, then you tune it at 45) to achieve the best in overall SQ and loudness.
You want everything to be in conjunction or as balanced as you can get your system (limits obviously come from the actual components you're using). The point of tuning is so you can get the overall best sound out of your speakers, while blending your highs/mids with the lows of your subwoofers. If there's no balance between the lows coming from your sub stage and the highs/mids coming from your front/rear stage then you need to reconsider if your tuning was done properly.
When you have the HU at max, that has to be the MAX you want not only your speakers to ever get but obviously your subwoofer as well. In the sense that you want to be able to display your subs complete power but being able to keep it equalized within the power limits of your speakers.
Technically speaking, you would describe that as the overall harmonizing affect of your system or it's fidelity.