Not saying you're wrong, but do you have anything to document the voltage levels CD's are recorded at? I'm curious as to what that exactly means.

I'm also not sure how you would lose sound quality SOLELY because you played it back at a different voltage than it was recorded at.

What higher voltage outputs let you do is get the signal above the noise floor. This is what is called signal to noise ratio. All the electronic goodies in your car make noise. Depending on the shielding, this can be loud or quiet noise, and can make it into your RCA's, where it looks like sound to your amp, which will reproduce it. Say this noise looks like a .05 volt signal to your amps. If your headunit only puts out .5v, this is only a 10:1 SN ratio. Kick the output up to 4v, and you have 80:1.

The bigger part of this problem is when the volume is down. Since 4v is a maximum at full volume, normal listening levels are much less than 4v. Since volume is a log10 function (not linear) 1/2 volume is actually .4v, not the 2v you were probably expecting. Take the .5v example, and your down to only .05 volts...right in the noise. This is why some systems have gain hiss...that noise you hear when the headunit is turned all the way down. Many headunits actually mute the output completely at very low volumes, so you can't hear this. Many test discs contain zero bit tracks...digital silence, that you can 'play' at any volume. This is where all the ugly noises you don't think you have surface. They are always there, slightly clouding your music, you just don't notice them. This is also about the only way to isolate noises coming from the headunit, as they will be volume dependent.

Now, look at the amp gain. You set this so maximum output comes somewhere near maximum input. This becomes the sensativity of the amp. The higher the input, the lower you set your gain, the less sensative your amp is, the less that pesky noise floor gets reproduced.

Back to your statement about playing back a 4v CD at 2V. I believe that in the absense of noise, as long as you match the gains on the amp to the output of the headunit, sound quality will be the same. Its not like your compressing the dynamic range by going to a lower voltage, as the peaks and valleys of the music are all relative. That being said, I don't know everything, and if you have data showing this, I'd love to see it.

Now, do 4V units really put out 4V, maybe. I think this would depend on lots of things, like the input impedance of whatever it is driving. Also, where is the 4V measured? At the board? At the end of the pigtail? Wherever, I doubt it will still be 4V at the end of a 20ft RCA.

In any event, more voltage is better than less voltage, provided you don't exeed the inputs on the amp. Even then, you'll just end up not using the top of the volume scale.

Toby