Hello @whyrichard , as usual, @ghdprentice is giving sage advice. You did say 'calm the shrill' so maybe we are agreeing, but glitter often does coincide with harsh, fatiguing overextended highs which at first sound amazing and then lead to listening fatigue. Its ok, I call the goodness in my system 'sparkle' and to me it means resolving but without glare or fatigue.
Looking at the newer Nagra streamer, it has USB and coax outputs. You need to pick which one is best for your DAC. But build materials of the cable do matter. For me, all copper cables are smoother than any I have tried with silver in them.
I have used excellent digital cables from Network Acoustics, and my favorite, a Realization from Kubala Sosna.
And yes, Digital Coax cables use an RCA connector, but the cable should be built specifically for digital.