Sonic differences between servers

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.  

@Ironlung makes good points and provides useful clarification, too.

And, I would agree with the last sentiment he/she shares: “Personally, I would avoid Roon altogether if SQ is actually your ultimate goal.  

I have direct experience w/ Roon vs other software going into the same hardware.  Files streamed via Auralic’s lighting DS application sounded significantly better than the same via Roon.  I dropped my subscription.  As an interface - Roon is great.  As for sound quality - not so much.

Files streamed via Auralic’s lighting DS application sounded significantly better than the same via Roon.

I'm just curious if the files were local (i.e. on a NAS or computer) or did you use a streaming service like Tidal or Qobuz?

In either case you heard the difference between the Roon player software (on whatever you were using to act as the Roon Core) and the player software embedded in the Auralic streamer. The Lightning DS app is just a control point software, meaning you made your song selections using the app, but the software doing the playback itself is in the Auralic (the App just provides control and feedback, not playback in other words).

I've had similar experiences so it's not surprising to see them confirmed by others!

I appreciate all of the feedback ( no pun intended).
It seems the deeper you get into digital, the more the marketing takes over and the truth gets lost.
I would drop Roon except it irks me to have to drop a "lifetime" subscription.  Another question for the crowd.  Has anyone heard the Wyred 4 Sound servers?  Besides being dollar limited, I have space issues as well and would go better with a compact foot print.
Thanks
M

on your criteria you might want to look at an OpticalRendu with fibreoptics connection to your router.