The final audio fidelity is dependent on the underlying audio hardware along with the Audio framework (WASAPI). Since Windows 10/Server OS shares the same framework (WASAPI), I don't believe one will sound better than the other.
Yes. Different versions of OS has different background tasks, etc and are quite likely to sound different, provided you take the effort to optimize them for audio. If you leave it stock then they are likely to be noisy enough by default that the changes may get masked.