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

It might be that in striving to create noise-free systems, solid-state and digital audio went below subjectively optimal levels. That suggests experimenting with adding slight amounts of white noise to virtually noise-free solid-state systems to see how that affects aesthetic evaluations. This could be done using a continuously adjustable noise source to dial-in a most-pleasing level, which will of course vary with the listener as well as the source and other features of the system. 

Thank you millercarbon for pointing me to the literature on dithering and your patient explanation! It would seem the whole added-noise topic has implications for all the tweaks audiophiles pursue to eliminate or modify the noise in the system (including that coming in from the music-signal source).
Re brettmcee on standards, I should expect no single ideal could be found, not only due to variations in source type and taste, but due to the huge variation in hearing (e.g., our frequency-response profile changes dramatically with age). So custom equalization of the noise level might be a new holy grail for system optimization - with the curve heavily source-dependent and changing over one's years. At best we might hope for reliable user-friendly equipment to do that kind of detailed full-band customization.

My belated thanks to jnorris2005 and atdavid for their critical comments warning of sales pitches (I suspected that one) and educating me about dithering vs pure white noise. Perhaps someone has built or will build the kind of variable source that would enable systematic study of white-noise effects on music perceptions.
Until then I'll just have to continue experimenting with the electronic volume equalizer in my processor (Yamaha CX-A5000 with ESS 9016 DAC), which to probably no one's surprise still makes stereo (as opposed to multichannel) sources sound best in direct/bypass mode (direct mains are Magnaplanar 3.4s driven by a Denon POA-2400A - truly antique stereo set but still sounds superb).