Agree with @andybob Assuming you hear a difference in audio players with all other variables being the same, then the only thing you can try (if you haven’t already) is to run RoonServer rather than the full Roon, which strips out the GUI and should reduce the amount of processing that is running on the PC. I provided some guidance on another thread explaining how to run Roon as a windows service, you could try that in conjunction with uninstalling the windows GUI on your server to see if that helps.
In the future, when the RoonSpeakers app is released, you could move the processing being done by the Roon Core (library management, codec processing, etc.) onto a different machine, leaving only the rendering functions on the playback PC connected to your DAC.
Or, as @Chrislayeruk mentioned, remove the PC from the direct playback chain altogether, and try a dedicated endpoint. Currently the only option is meridian, but in the future I am hopeful that something like the Sonora Rendu will be updated to support the RoonSpeakers protocol.