To address your question, I will tell you a few things about gain first. Gain is used to level-match the voltage from the headunit to the amp you are using. If you aren't real familar with electricity, don't worry. Getting the jist here will do you just fine. The best way to think of gains (some amps actually use this term instead of gain) is as an amplifier sensitivity level. In other words, how sensitive to the input voltage of your headunit do you want your amp to be, so that these two devices agree on volume level and output. So, the lower your output voltage on your headunit, 2v preout for example, the more sensitive your amps have to be to match the level. Think of it like this; if the output is weak from the headunit, the input must be strong from the amp. So, a 4 volt preout would allow to keep your gain setting lower than a 2 volt and a 9 volt, lower than a 4 volt. Now, this being said, generally, the higher you gains are set, the more sensitive the system becomes to noise. More sensitivity on the amps (higher gains) means that your amp is more sensitive to all electrical signal. So, the signal thrown off by your alternator and by other powered devices that are picked up can be transmitted through your speakers. When you play a CD, the headunit interperets the recorded signal and a signal from the headunit travels through the RCA cable, into the amp, out of the amp into speakers, out of the speakers and into you ears. The result is, you hear music at a particular frequency. Well, electronic devices around the inputs of your amp which are picked up faintly through the input without direct contact are also transmitted that same way and those too come out of your speakers at some frequency. Usually, distorting the intended sound of the music. So, to recap, obviously higher preout signal voltage means lower gains which may translate into less noise or a lower noise floor. However, this all depends largely upon your amps. Your noise floor only becomes an issue when your amps pick up and transmit that noise. And whats more, the only time it matters to you and I is when the transmitted noise is audible. So, you may have an amp that can handle it's sensitivity being high and still not pick up the interference around it. Therefore, it wouldn't make a difference if you had a 2 volt preout or a 4 volt preout. Some amps, would benefit from a 9 volt because they are really susceptible to noise at high sensitivity levels. The ultimate answer is that it truely is situation dependant and one really cannot determine the effect of preout voltage without factoring in every other aspect of a particular system. Things even as trivial as what material your tweeters are made of matter, whereas titanium tweeters will make most everything audible so even the subtlest detail of noise will be an issue. Basically, everything 4 volt and higher are typically not a problem. Some switching power supply headunits with preout voltage upwards of 9+ volts are usless to a large number of amps which cannot even accomodate them anyway. The difference between 4 volt and 2 volt on a descent amp is not really much to worry. Especially when you account for the one factor I haven't even mentioned yet - your hearing. Not everyone, in fact a relative small number of people can even hear subtle details of noise and distortion. So it doesn't become a problem for a lot of us anyway. And the difference between 6 volt and 4 volt is even less noticeable usually and not all amps will even make efficient use out of the 6 volts anyway. Hope that helped a little