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

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.

Sorry, forgot to add, my system is Cary Audio SLI-80 Signature integrated, Chinook Pre-amp, Linn Sonedek LP12, audio Mirror Tubadour III Nos Tube DAC and Fritz Carbon 7 speakers. 

So what would happen if you buy a stand alone white noise generator and have it playing at low levels in the background while you listen to your system?

Seems like most of them are under $50. Maybe $200 for a deluxe version. By audiophile standards this is the cheapest tweak you can imagine

Hiss and pops and clicks and other forms of tube and record noise are not white noise.

If the signal path is absolutely clean then there would not be any need for the added white noise.  But alas ... our digital gears are full of "correlated high frequency" noise.  Some DAC intentionally have "white noise" added to mask or lessen the effect of "correlated digital noise".