Digital Joseph we are a Lumin dealer so our experience with the X1 is far greater than the DCS product.
First the Bartok also has a built in streamer so currious to the mention of the U1?
In our tests we have found most dacs' sound better being fed by an Innous Zenith or an Statement., as the data is cleaned and repacketed before it gets to the dac.
Both of these players are pretty awesome. The Lumin has one of the single better sounding digital streamers on the market in the X1 the sound quality is really amazing, in our 30 years of working with high end digital the X1 is one of the most memorable streamer/dacs out there and the reviews have been off the charts.
The Bartok has digital inputs and a headphone amp which are pretty cool. Our experience with previous DCS the DCS sound was more analytical, in the case of the Bartok it is a DCS starter product, in the Lumin X1 it is their flagship and Lumin's enginers have thrown everything they have at building the worlds best streaming dac.
Many years ago we had a client trade in an $18 DCS CD player dac for the Lumin A1 which at the time cost $7,200.00 and the Lumin was much more musically involving, the ring dac sound has always been hyper clean and detailed so unless they have managed to warm it up that may be an issue.
If you have specific questions at to why adding a separate server to a system may be a good idea you can always PM us.
Good luck in your search.
Dave and Troy
Audio Doctor NJ Lumin and Innous deaers