Hi and thanks for your replies. I have to go away for a day or two so can't make many more experiments.
I have set up Jack on this machine before, using Fedora and Audacious and it played excellently with (my own design) USB DAC which uses and XMOS receiver.
I have another machine 3m from me, running Mint, which runs Jack and Audacious, and Kodi perfectly with the same DAC.
My wife's machine, which is 1M away from me ran Fedora with a real-time Kernel from Planet CCRMA when I was using it as a HTPC, using various DACs.
On this machine I also tried Clementine and DeadBeef, both play, but clicks and noises, which I assume are xruns, on hi res files, no matter what settings I try.
I have all the gstreamer plugins installed and, in any case, the fils in question are pre WAV PCM files, no format conversion is taking place.
The other issue, as I said, is unrelated, but irritating: sometimes I will return to find my machine asleep, but it is unrousable and the HD LED is rhythmically pulsating. Only a hard reboot will fix it.
Again, I will say that I have had Mint and then Fedora on this machine for over a year and had none of these problems.
I'll have another look at the noise issue in a day or two.