Mac mini or Synology (or something else)??

[Axel]

By the way, can a Qnap running jRiver be a good idea as an only source?

Yes! I use a QNAP HS251+ running JRiver and controlled with JRemote on an iPad. I used to run this straight into the D visa USB and it sounded great. I now run the QNAP into my Mutec and on to the D via AES/EBU which improves the SQ even further.
This is not my only source, however. I also use a Sonos Connect, a turntable and a CD player.
If it came to it, I could easily be happy with just the QNAP as my only source. It never fails!

Great!

Is it also possible to set JRiver to "play from memory instead of disk"? This since it seems to improve SQ, at least for me.

But just to make things clear( maybe I'm an idiot) but if I buy the HS251 (must it be the "+"?) I can install some kind of operating system like w10, install JRiver and use my iPad or Android as a remote, connect it via USB straight into Dev - and that's it? No need for any computer during playback?

Erm, I think you can play from memory, but can't be sure. The plus has an extra Gigabyte of RAM and  a more powerful quad core processor and I'm not sure whether the 251 can run JRiver. It won't run Windows, but has Linux built in. Just install JRiver from QNAPs website, then you can go straight into the Devialet via USB. No need for a computer to be running during playback although you will need a monitor/television connected via HDMI to set JRiver up. You will also either have to connect a mouse and keyboard or use Qremote (which is what I use) to input data.
If you need any more advice, please ask. Alternatively, there is a JRiver Wiki, which has comprehensive help files.

Edit: I've just checked, and you can play files from memory although I've never needed to use this setting.