There was obvious noise in my system when I was using an old nettop pc > USB DAC > tube buffer > power amp > speakers.
The dual core Asus nettop (EEE Box EB1012) ca. 2008 turned out to be the problem.
Using a newer Raspberry Pi 3 using the *same* USB DAC is much, much quieter. As a bonus, the Pi is also fan-less.
What a relief, and more evidence that each and every part of the chain ought to be subject to scrutiny!
At first, I thought it was every other powered component. Silly me. Lesson learned.
