I think it would be hard to practically tell the difference between a server source versus a recent even decent quality CD player through the same DAC in most cases (its hard for me in my rig), but technically the server technology no doubt has the advantage in general over optical read source devices alone.
I think you raised an observant point. Servers and the quality/consistency of digital music are still maturing, whilst cd players and cd media are highly evolved (eg: xrcd24,dxd,sacd). So whilst technically high rez downloads & rips have the advantage of buffering music to eliminate read errors, high end cdp's can play a range of audiophile cd's through what are typically better dac sections & connections (eg: neutrix xlr). But there are also a few cdp's which buffer cd's like the Boulder 1021 ;)
