750$ Intel NUC vs $6000 Aurender N200: I don't hear the difference

The following is not a definite root cause, but just an idea of what you could be experiencing.  You are using very high efficiency speakers (96 db) which have a silk dome tweeter (soft dome tweeter).  The soft-dome tweeters are sometimes more forgiving and not as revealing or transparent as other hard-dome tweeters.  Additionally, the tube in the Nagra dac -might- be softening the sound and resolution as well.

It could very well be that your system is not high resolution enough to show the difference between the NUC and the Aurender N200.  If you are using a linear power supply on the NUC, then it will be even harder to hear the difference.

Once again, just an idea of what you could be experiencing, but hard to prove.

lord melton we have tested these servers under identical conditions

same everything just swapped usb cables pretty scientific

also the reviewer ed seally has tested many servers

Your conclusion makes sense to me.  Sever has one job, deliver bit-perfect data to your DAC.  

Use your ears.

I own the Aurender n200 and in my testing I found there is definite difference in sound quality between the usb and coax output. IMHO the usb output is all around better than the coax ouput. A fairer test would be comparing the nuc and aurender usb outputs to your dac. 

Plus Qobuz sounds better than Tidal. Tidal is thin sounding and a tad harsh in the highs compared to Qobuz.  Then we see different cables are being used. This impacts the sounds greatly. Most likely different power cables and footers are also being used.  What about burn in time on the new server? So many variables.