Well, I spent a good couple of hours trying to get Roon to see my DOS 2 Phantoms as a UPnP renderer only to eventually remember that Roon doesn’t support UPnP. That’s why we started down this SonoreUPnP Bridge route in the first place. Doh! My memory is good but short. 
Anyway, I reinstalled the SonoreUPnP Bridge on my sonicTransporter, pointed it at the Phantoms, set the output to WAV and presto, music! It now successfully plays 16/44.1, 24/96 and 24/192 and certainly plays longer than 45 seconds - but unfortunately it still cuts eventually. For instance I tried a Haydn Symphony in 16/44.1 from Qobuz. On track 1 it cut after about 2:14, track 2 1:57, track 3 1:54 etc. So progress, but not there yet. Still, as the Flight of the Concords so insightfully observed “two minutes in heaven is better than one minute in heaven”…
Every cloud has a silver lining however and in this case it is that my Devialet “hand cream” Remote, which resolutely refused to pair with my Phantoms on DOS 1, is now working with no problem. Yes, I now have a working knob!! 
@ogs @Henry_McLeod @simon_pepper @grossmsj @Jesus_Rodriguez fyi.