re: cable noise
It's actually pretty darn easy to make 50nV noise in unshielded wires (just bend them, or let them vibrate, or wave your hand in front of them, and the capacitance of the wires will pick up the moving charge and generate a current in the 10nA range easy) but the real question is how much noise will be in an audible frequency range. Answer is: basically none, other than 60Hz pickup in some unfortunate systems. So as far as your ears are concerned, clean digital audio frequency signals are pretty easy to generate, digitize, undigitize, transmit with no noise added.
What I don't know is whether audio engineers add imperfections to the raw digital stream to make it sound better. In digital graphics, there's something called "antialiasing", where you try to compensate for imperfect sampling by some kind of interpolation or smoothing. I assume that you can accomplish this in audio by taking the 44kHz signal and doing a low-pass filter on it, say a nice smooth attenuation of all signal above 20kHz.
So maybe a $30 CD player has perfect fidelity but poor antialiasing.
I'd like someone who knows to chime in here, but I bet that a lot of the difference between speakers is in the way they filter out the very high frequency sounds, and that the supposed harshness of Axioms is because they reproduce any harsh, unfiltered digital signal from that $30 CD player with excellent fidelity (garbage in, garbage out), and the supposed warmth of high-end speakers and players and tube amps is because they filter out, in a manner pleasing to most ears, such higher frequency junk.
