I can't really comment on any noticeable difference in sq, but I know my helix dsp will only accept 4v max so keep that in mind if you have a helix dsp or thinking about it.

 

I think it really comes down to what your other equipment accepts, and you can always turn down the 5 volt output if need be, but 4 volts is probably enough. Just get what you like.

 

So is the theory behind this have to do with maximizing the source signal’s volts because they are cleaner than the output signal - which would require a small bit more of amplified gain to make up for the unused volts? Do input volts not have a significant noise floor or something?

 

So is the theory behind this have to do with maximizing the source signal’s volts because they are cleaner than the output signal - which would require a small bit more of amplified gain to make up for the unused volts?

Yes

The theory is more input voltage = less gain = less noise for a given output.

 

Higher voltage increases the signal to noise ratio. The noise that the cables pick up is a certain fixed level, so if you can have a higher voltage level, the noise will be a smaller fraction of the signal. You’ll be gaining up that voltage to a higher voltage to your speakers, and the noise will receive the same gain.

 

Keep in mind, a 5v headunit is not outputting 5v unless you are cranking the HU to the max. You will never listen to 5v output. IOW, its a non issue.

 

While I agree with this, it's worth noting that the whole volume range will be 25% higher voltage with a 5v output than a 4v. To that end, a 5v output has significantly better signal to noise at lower volume.

 

The higher the better , the higher voltage your head unit can put out the lower your signal to noise ratio will be (a good thing ).