OK, I understand that to perform to potential, better aftmkt speakers need more watts. So we replace our oem head unit and add an amp providing anywhere from 40w to 100w per channel.

Aftmkt head units also offer a diversity of pre amp output voltage, anywhere from 1v to 5v. I've heard the improvement when higher wattage drives the same pair of speakers, but what is the advantage of a the head unit's higher pre amp voltage output?