Could this be one reason tubes (and perhaps records) sound better?

I'm not sure tubes sound better - it is just a preference.  Noise is often used in imaging to improve resolution, but in sound it is likely to improve dynamics.  Distorted guitar sounds more dynamic than clean Jazz guitar at the same level.  Added noise is a form of distortion.

@millercarbon  I believe that we got used to particular system sound, different from reality.  We know that in concert the sound is different but at home we like what we hear.  When I replaced CDP with Benchmark DAC I had impression that sound is strange.  It sounded too clean - like something was missing.  One person stated that he can hear all instruments separately, but prefer to hear them as one (sound blob).  We need to learn to listen.