I went back to the CXC earlier, but the fault is still there, so I looked to see what else I had.
Then I remembered I had a couple of NEC Multispin 6X CD rom readers. So I dug one out and installed it. It's archaic in needing a caddy for the CD, but it does have a SPDIF out, so it is usable with my Beresford SEG DAC.
Everything seems OK at the moment.
