Tubes are nothing more than a band aid for very poorly designed CD players. They add 2nd order harmonics to make it more pleasing to the ear.
I see your point with poor quality cd players. By design one could add 2nd harmonic distortion hoping to "smooth out" the sound. I think most good players attempt to limit any harmonic distortion by design even though there is not much room to optimally design an analog stage that limits other forms of distortions.
I prefer to bypass the entire analog/mute sections and run directly from the digital to analog chip itself for better fidelity. The output is still approximately 1 volt rms out so no huge loss of signal.
Cheers.
