thank you very much @john 
hope developers come up to a fix: looks it’s something related to Integer Mode, not to Exclusive Access, as in Audirvana there’s no way to disable the latter
@Gigatoaster issue is that on Mac OS Roon can’t start playback to an UD-501 connected via USB: UD-501’s display keeps flashing “USB Ready” and nothing plays
Then, if by mean of some tinkering, you succeed at starting playback… Roon chokes again as soon as you try playing a track which is lower resolution than last successfully played one