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

Probably should move this comment to another thread but not sure where. I had the same experience as kijanki states above. Relative newbie, I just bought my first external DAC, an old Berkeley Alpha DAC Series 1. My immediate impression wasn't what I heard but what I didn't hear. I was playing chamber music and I distinctly detected the silence spatially between the seated musicians. Don't know how or why, but I definitely noticed it.

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).

lesdomes,
There are white noise generators readily available with volume control, they are used as sleep/work aids. If you have an analog mixer, you could also mix white-noise from a laptop to whatever else you were listening to.

Personally I would not recommend intentionally generate white noise in your listening environment.  There is a difference between what happens in the laboratory environment and your own.  Just because they said so, does not mean it's true.

... which is why experiments are recreated, for validity. I am struggling to see any merit in your advice to someone who wants to experiment.