well, generally speaking the higher the voltage the higher you are off the noise floor...
so if you area adding extra amps, (subs, seps) then the higher the voltage, the celarer the signal youar esending to the amp to be magnified and then in turn be sent to the speaker.....
basically, better SQ, less static... most of the time...
once you get to ta certain point, like 5V i think, you are so high off the noise florr that an 8V preout wont make any difference, except ion the wallet...
4V is great if you want to no hissing...