RCA outputs

Check the LOC manufacturer for suggestions, but generally you start with the gains at full.  After setting amplifier gains to match the deck output (setting gains by listening for clipped signal), listen to some music sources.  Adjust the LOC gains down a little and listen to hear if the tonal quality improves...if it does, adjust those gains to the best sound.  Afterwards, go through the amplifier gain-setting steps over again while leaving the LOC alone.  But there's an IF...

If you can't properly set the amp gain because there is no play at all to turn the gain up, adjust the LOC gains down to about the midway point and try setting amp gains again.  You'll know there's no play in the amp gain adjustment if its all the way down and clipping is heard.  Then, if those midway positions work and you are able to turn the amp gain up a little, tweak the LOC gains upwards from there, resetting amp gain after each adjustment of the LOC.

The LOC gains allow for an adjustment of input voltage.  At full, it allows the full voltage from the deck to the amplifier, and in most cases the voltage is not too high for the amp to handle.  But an OEM deck's speaker level output is higher voltage / lower signal current than an aftermarket deck, so the voltage is typically higher than you might expect.  As long as the voltage isn't too high, setting them at the highest setting available will allow for maximum signal-to-noise ratio.  Don't settle for the old "just set them to the center and be done with it" route because you need to get every bit of sound quality you can out of that high level output signal (which is not as clean as a line level signal).

I recommend marking the LOC gains position in case they are accidentally moved from where you set them.  I use fingernail polish (red, my favorite!) for this.