(Todd H.) writes:
I may have missed it in the details...but why not use the fiber that
was working fine?

No easy way to completely rule out grounding, or to rule out a problem
with the circuitry driving and reading the co-ax, but if these
problems go away using fiber, seems that's the ticket....

Oh duh. You changed driver equipment. Okay.

You've got several possible things that could be the problem to narrow
down. Could be that the SPDIF output of your card isn't clean.
Maybe it's got a problem. Independently test it with other
equipment. Maybe your co-ax SPDIF cable is pooched. Maybe there is
a ground loop of some sort interfering with the digital communication
(though if you can't hear one anywhere, that seems somewhat
unlikely). Maybe the co-ax SPDIF input circuitry in your DAC is
currently or never did work but the optical was always fine. Basic
troubleshooting stuff in isolating a piece of the chain at a time from
the environment is the only way to narrow things down I'm afraid.
Sadly that's quite a bitch in an installed environment.

Best Regards,
--
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