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

It comes from experience. I have never experienced anything in audio that could be 1) Verified , and 2) Not measured.  Usually #1 eliminates almost everything, and when it does not, #2 takes care of the rest. Every single time where I have heard, or experienced a verifiable sonic difference, there was also a measurable difference. Not just measurable, but above what are accepted limits to hearing sensitivity/discrimination, and significantly different.

I am not talking single point, THD either, but full scans, i.e. THD across frequency and output power, IMD with 2-3 points, and multi-point at multiple powers. Then again, just taking two time correlated streams (digital source only of course) can be highly revealing.

I think we both have respect for isolation, at least for the analog. I do believe a lot of the claims of "noise, RF, pick your poison" getting into the DAC clocking is for the most part suspect. If you know RF, you know that there is more RF coming off that trace 1/2" away then you are going to pick up externally. If the people who made those claims knew depth of D/A design, they would be far more worried about the D/A analog reference :-) ... but also not difficult to regulate for audio frequencies and eliminate noise.

I have convinced myself of things I heard that I later proved were not there. You are so hyper focused that something must be there that you convince yourself there is.

OP could be experiencing sin of omission deal, nuc vs. sin of commission, Aurender. Now is that sin of commission due to Aurrender or some other part of system?

One often doesn't perceive omission as a sin, and it may be preferable in some cases, perhaps in a lot of cases.

In terms of your problem logging in to Qobuz, your firewall may be blocking their site. I’ve had this problem and had to have Netgear tech support unblock it in my Nighthawk Wifi 6 router. My thoughts are that with a tube based DAC you probably won’t hear the difference between the lower noise floor of the N200 over your NUC, especially while using ROON because the tube noise is above the noise floor of the least capable source, namely the Intel NUC. I recently moved from a similar setup, got rid of ROON which has it’s own SQ issues and am now using a high quality streamer/server made by NOVATRON that has built in storage and runs on a linux custom processor and has exceptionally low noise characteristics, a highly accurate clock plus exceptional physical build quality with much attention to detail, yet only costs about $2K . This allows my (slightly) below premium grade Gustard X16 SS DAC (connected by USB)  to really strut it’s stuff. One would not have guessed a streamer/server could make that much difference but it really does. 

OP, do not be led to believe there is anything wrong with your hearing or impressions. You are not the first to report hearing zero difference between servers. I suspect many owners of high cost "audiophile" servers will denounce such a finding as nonsense.

I have long been skeptical of high-end servers. Some claim to use custom motherboards, but most use off-the-shelf processors from AMD and Intel. Their proprietary operating systems are touted, but most are based off Linux. I also see a lack of accommodation for upsampling, which requires a lot of processing power. Most servers are simply not powerful enough to do upsampling, at least beyond DSD128. Support for software like HQPlayer is also lacking.

If one is willing to do DIY approach, a SOTA server can be built for fractions of the cost. Take a Mac Mini or Intel NUC, boot it from Audiolinux (which has native support for Roon and HQPlayer), add a LPS, and connect it to your network with ethernet.