Level setting, done by ear, is more art than science. It can be done using an oscilloscope but since few people have one of those laying around we’ll cover doing it by ear. Basically you want to start with the first component in the chain (the head unit) and work your way to the last component (the amplifier).
1. Start by turning all of the input level adjustment knobs (gain controls) on your components fully counter-clockwise (to their minimum setting). Set the tone controls (bass, treble, loudness) on your head unit to no boost (bass and treble level = 0 and loudness is off). If you have more than one RCA pair you will want to set each gain adjustment separately. Make sure your fader and balance controls are set to the channel you want to adjust first. This can be an individual channel if you have individual gain adjustments or a pair of channels if you have one gain for two channels.
2. Next set all of your equalizers settings (if you have an equalizer) to the center (detent) position so they produce no boost or cut. What we want is as pure a signal as possible.
3. Put in some good quality source material, preferably a CD with strong output and a clean recording. Hard rock would be a bad choice here. Try something cleaner, maybe acoustic, that you’re familiar with.
4. Turn the deck’s volume up slowly until you begin to hear distortion. When you hear it, stop and back off slightly until you no longer hear it. If you don’t hear distortion, even when the volume is all of the way up then you have a quality head unit. That’s what we’re looking for.
5. Now with your head unit at maximum undistorted volume move on to the next component. Adjust it’s input gain until you begin to hear distortion. Back off slightly.
6. Continue this process until you have all of the components in the chain at their maximum undistorted level.
7. When you reach the amplifiers you may need to wear earplugs to adjust them to their maximum level. As before, turn up the gain until you hear audible distortion. This should be audible even with earplugs in. But honestly, if you have to wear earplugs to listen the distortion level is probably not a factor
That’s it. Play some music and verify that everything sounds right. Congratulations! You’ve just learned to properly adjust the settings on your car audio system.