That depends on whether the rendering stage of rhe ND555 with uPNP via network is better than that of the Core as output via SPDIF. Quite apart from the presently unknown comparison purely of the two rendering stages, networks introduce additional variables, which can include greater susceptibility to other sources of degradation.
naim demonstrates its streamers with the uniticore in upnp mode at shows, because they consider it to be the best. In spdif, you are using only the dac section of the nds, ndx or nd555.... if you want to use spdif with the core, better is to connect a separate dac, like ndac or chord dave.
Is the use of uPNP at shows for quality reasons declared as such by Naim? Or might it be for ease of control, or more likely, perhaps, because when demonstrating an all-in-one streamer one would do so with a network simply because it is being marketed as capable of running on a nework, and the Core is but one example of a serve. Demonstrating with the Core-specific SPDIF feed would be something different, focussing on the Core’s capabilities rather than just the streamer.
I seem to recall Phil Harris stating something to the effect that he could hear no difference between Core as renderer and NDX.
in spdif mode you don’t use the streaming part of the ndx or nds, so it’s not very logical. But you can do it and show the capabilities of the core in that way, i agree with you innocent bystander. Just using only a specified dac is more logical.
As for sound quality, perhaps a core/ dave can compete with nds/ core in upnp, it must be in the same league, just different.
For upnp vs spdif, i could compare at home with my serve/ linear ps( very similar to the core): with a good network isolation and quality lan, vs spdif with dc1, the upnp was much better for me, at the first seconds of listening.
I know you have the dave, why not try to borrow a core and connect in spdif, and compare to your mac mini?
what means » innocent bystander « , if i may ask?