Can you really hear the differences in using either..? I know 6V is a more clear signal, but in general, what would I gain..? Does Alpine have an 6V HUs..?
It shouldn't matter at all, and generally doesn't.Line level voltage is just a bit of insurance against one particular type of noise, and it's not a technology to eliminate it, but rather drown it down a bit, if it is experienced. It's not a solution to the noise, per se, but rather technology to make it more tolerable and less noticable.
It's rather like octane ratings with gasoline...
If you don't suffer from pinging and detonation with a particular grade of gasoline, buying a higher octane gasoline offers absolutely no benefits.
Likewise, if you don't have noise - and specifically noise that's induced into your RCA cables south of the head unit or line driver - at a particular voltage, a higher voltage won't buy you any benefits.
It is important to feed your amplifier a voltage that it can handle, that is, a voltage that's within the adjustment range of your gain control, because that's what it's for. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
If you don't have noise with a 1v line level HU, then 2v, 3v, 4v won't buy you anything.
It doesn't have anything to do with signal quality.