As for the output voltage.... Loud is not as much a priority, as SQ. You are correct that output voltage doesn't increase sound quaility...in, and of itself. HOWEVER...2V outputs are derived from the car's 12v supply, which itself varies with the alt/battery/load. 4V,5V,8V outputs, use switching power supplies. These switching power supplies use a constant rail voltage to derive the higher output voltages. In short, they have clean output up to the rated voltage. 2V units will ALL clip the output signal near full volume....some as much as 20%!! Don't know about the rest of you, but I could do without unnecessary clipping induced into an otherwise "clean" system. Second, the larger the "clean" signal voltage to the amp (or other downstream component), the less noise you will pick up at the downstream gains. Doesn't matter if I can "match" the gains to the incoming signal, or not!! Your preAMP stage AMPLIFIES the incoming signal, AND any noise that came along for the ride.
Small amount? Maybe. But, all vehicles are different. What might be nothing for one vehicle, might be intolerable on another. If nothing else, I'll just take the 4V output for the lack of clipped signal at full volume.