That Nagra link on the back of that streamer is the same proprietary PLINK connection from Playback Designs. I have the prior version that will work with the latest PBD DACs. I actually had mine up for sale for $2k but changed my mind and decided to save up for a top end PBD DAC. My PBD Strem-IF will work with Nagra DACs.
PBD has a newer version of my unit for $3k. That new one has PLINK and USB while mine has SPDIF and PLINK. I prefer my connection options.
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The SPDIF sounds amazing into my Benchmark DAC3B. My frame of reference is the Lumin X1 streamer and Sonore OpticalRendu. I had all 3 side-by-side and was shocked that the PBD Stream-IF was hanging with the fibre optical of the other 2. The PLINK should be even better compared to SPDIF.