I now have the Fan-less NUC running on a nice linear power supply and I do like the sound better. Basically I perceive a lower noise floor. I did not think it was possible anymore, but I do like it better. Tried few days with Quboz and local files sitting on the core ssd drive.
Details: The NUC is connected to the network. All of my network components run on linear power supplies (modem, router, switches). I also use fiber for isolation and long distances runs.
I fear that this finding unfortunately supports the 'everything matters' camp in the digital domain. It also means that a lot more money will be spent in the future