If the job of the server is to send the digital signal to the DAC
That is not the job of the server, that is the job of the player/renderer (an application layer software) and it's associated hardware interface capabilities (i.e. USB, SPDIF, AES/EBU, Ethernet stream).
A server does what it implies. It serves files to any number of potential endpoints, each of which can have their own unique player/rendering software.
With respect to Roon, a Roon Core contains both the server and the player. So your music "server" in the case of Roon, is also your music "player". The Roon Core sends real-time audio playback out over your LAN to a Roon certified endpoint. The server/player/endpoint scenario with Roon is therefore a bit more complicated, because essentially the server and the player are the same device; the Roon endpoint has the ability to "capture" the real-time audio playback happening on the Core and sync the file playback to the DAC, over the network. (What they call RAAT, which is basically a glorified version of AirPlay given the above analysis).
In this case the difference between endpoints is going to be related to how they interface with the DAC, since the player is identical between Roon endpoints.
I suppose what I am trying to point out is that because of the confusion surrounding all of these terms and products, speaking about differences in sound quality pretty much ends up becoming more or less a moot point when it comes to "servers", especially if you are using Roon. Personally I would avoid Roon altogether if SQ is actually your ultimate goal.