Nagra tube dac owner here. A few thoughts:
1. the digital part of the Nagra -- with its DSD upsampling -- does a lot of lifting. I would say server/streamer matching is much less important with the Nagra, than it is with other dacs I’ve used.
2. the analog output stage of the Nagra is one of the best in the business. This is of course what Nagra is legendary for. Analog, tape specifically.
3. notwithstanding 1, there is a clearly synergistic streamer to pair with the Nagra, and that’s the Playback Designs MPS-X. Designed by AK, who designed the digital module in the dac. Just a streamer, so you can continue to use your NUC as the server. This replaced a Grimm MU1 in my system.