lets say you have a 1500 watt amp rms, and you have an input voltage of 2v thats is usable when its cranked, so not the amp has to bring that voltage up to 54.77 volts to have it output 1500 watts to a 2 ohm load, now that basically put it to having to increase the voltage ~27x the original input, now if you have a 4 volt preout into the same thing you will be only amplifying ~13.5x the original signal voltage, so in theory having a higher preout voltage would considerable lower the amount of noise an amp will let into the output signal, the higher the voltage going into an amp, as long as the amp supports it and its a clean signal that doesnt already have noise going into it, the better, you can think of an amp as just a voltage increase in all reality. Now if your running 7-9v's like I may be doing then your amp will increasing it about 7x, but that all depends on if my line driver in my 30 band eq will output a clean signal at those levels
Also another thing, if you put a line driver after a headunit, and there is already noise in the line, you would be amplifying any noise with it, same as the amp, the amp would amplify any noise in the line the same amount, so if you have 2v preouts, and there is noise, you would be amplifying that noise by 27x, that would be noticeable, now that all comes into your decks output clarity and any signal leakage from the RCA's, thats why higher preout voltage is wanted, because the amp will introduce less noise with the gain not having to be set so high, and the already small amount of noise in the signal will not be amplified as much, as every signal will have a very small amount of noise, sometimes not even hear-able, but still