I believe you will be able to understand, after watching the video linked below, that the CMRR of a differential amplifier will only reject the noise signal that is picked up in both wires equally (the only signal that is common mode) and will not reject any of the signal that is not common mode (the music signal).
I believe that if you really think about it you will be able to see that none of the input signal (the music signal) can be common to both wires but instead the music signal is differential between the two wires.
What we (meaning you and I) call music signal starts it's life at the diaphragm of a microphone.
Take a look at the circuit of a u47 microphone. The circuit is single ended. The music signal is the difference between the output of the tube and ground. The signal goes to a parafeed output transformer through C2 to the primary of the output transformer as a single ended signal. There can be nothing common mode up to that point. The other end of the primary is grounded.
The secondary of the output transformer is not connected to ground so the music signal is now the difference between the two ends of the secondary.
The two conductor shielded cable, with the shield connected to ground for shielding purposes only, connects the music signal to the mic-pre in what is called a balanced connection.
The mic-pre input transformer has a primary that is not connected to ground and will only respond to the difference between the two ends (the music signal).
Any noise picked up in the wire run between the microphone and the pre-amp will be equal in amplitude in both wires (common mode) and that input transformer will not respond to signal that is common and therefore rejects it (common mode rejection).
So the noise that is picked up in the wire run is common and rejected while the music signal is not common (it's differential) and is therefore not rejected.
Having said all that, please understand that I am not trying to promote push pull amplifiers.
I, like you, believe that single ended amplification is the better way to go but for completely different reasons than the ones you stated above.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
