When you turn up the gain on the amp, you do so to match the pre-out voltage on your deck. The input voltage of the amp goes downward as you turn it clockwise (turn it up). Thus, when matching the input voltage of the amp with the pre-out voltage of the deck, you don't need to turn the gain up as high for a higher voltage pre-out. The further you "turn up" the gain on the amp, the higher the distortion levels are. All of these things that I said are FACTS.
Now, whether or not your ears will be able to discern the difference in distortion between, say, 1/4 gain and 3/4 gain is subject to the individual. I myself can EASILY notice the difference. Here's a really easy way to tell: Bring your amp in the house, and hook it up to a car battery (or a battery charger). Connect it to a pair of speakers. Connect a source to it, but don't play any music. Start with the gain down, then turn it up. Notice the increased distortion? That static-y noise? There's proof for you. The distortion is most certainly less noticeable when music is playing, but it is still noticeable to a good ear.
that said. anyone who is saying preout voltage matters. answer me one question.
why is it that a lot of the old school amazing sounding decks have 2mv pre-outs?
That would be because they used line-drivers frequently back then, instead of high pre-out voltage decks.
Wonder why we rarely see line-drivers in SQ installs and more often see high pre-out voltage decks in our current time? One thing they failed to notice when they started toying around with line-drivers was that by adding another electronic do-hicky in the system; which required separate ground, power, remote, and rca inputs; they were introducing further noise. Sure, it's possible to eliminate that noise, but that requires a lot of time, dedication, experience, etc.
Not to mention that it's more convenient to have a deck with high pre-out voltage than having to wire up a separate line driver. It became the obvious choice.
I'm sure I'm going to get a lot of "tl;dr", but for those who read all this, I'm sure it will clear things up.
CLIFS:
These are facts - More gain means more distortion. Higher pre-out voltage means the gain is lower when the voltage matches. Thus, higher pre-out voltage means lower distortion, ceteris paribus.