Thank you for the wishes. I've got three days of temperature to 38,5°C, nose sometimes running and other times locked, cough and a nasty sore throat but I'm getting back to normal every day.
This morning I feel REALLY better because... lifting the ground on the AC side of the NUC power supply did the trick!
I'm not getting
any noise at all even from the RGB (RCA round) cable and even if I cranck up the Head Box HP1 volume. Not needing the balanced lines any more I removed the Mackie from the chain and now have only:
stereo unbalanced (RGB) from interface to Rotel stage input + stereo unbalanced (RGB) from Rotel zone 4 out to Head Box input 1 + headphones on Head Box HP1 output.
I also tried back the RCA flat from Rotel out to Head Box in and the difference between that and the RGB one is minimal considering that the flat one gives a tiny little noise only with the HP voulme crancked all up (I usually use it between 9h and 12h).
So I consider this side of the "noise" problem, solved and I heartily thank everybody for the help and the patience.
To the mods, if you think it could be useful, consider adding the word "solved" to the title.
After that, making some other test I noticed that connecting the audio out of the DESK PC to the input 2 of the Head Box, with a stereo flat RCA <1m, I can hear a little noise with the Head box switch on ch 2, but the same noise is a little louder when I switch the Head box to the channel where there is the cable from Rotel.
To me that noise sounds a little like the one of a very distant ultra light plane...
It's not comparable with any of the sounds you can generate here:
https://mynoise.net/NoiseMachines/50HzHumNoiseGenerator.php
Changing visualization from an almost totally white page to an almost totally black one on the main screen of DESK PC, the noise changes frequency and it seems that with the dark one it also gets a little louder.
The monitor is an old Formac one with, I think, a fluorescent lamp as a backlight source.
Thanks.
fabio