Is the Arcam the rplay? That apparently only has coax out, very odd. It does raise the possibility that the adapter that you are using is not allowing a high res stream, which was your original guess.
see if you can attach a computer as above and if it plays high res via usb. That would confirm that the connector is the culprit
see if you can attach a computer as above and if it plays high res via usb. That would confirm that the connector is the culprit