My 2 cents…
I do not have NOS experience outside the Musician Pegasus, which in NOS mode sunded “dull” compared to OS. But, if Musician adopt the same Denafrips approach, its NOS is not really NOS but OS with all parameters set to 1.
The Pegasus was very easy to listen for prolonged time, probably due to the “sweetened “ upper midrange and treble (i.e. rolled off).
I listen to the D70S for most of my available time with zero listening fatigue, aside badly mixed recordings which I cannot listen for more than few minutes - but it’s a recording fault, not dac’s.
When comparing some selected tracks between the Pegasus and the D70S, I was able to detect some more peaky treble passages with the D70S compared to the Pegasus. But, first, outside that peaky 1% the D70S was vastly more engaging than the Pegasus. And, second, as I stopped A-B-ing between the two dacs (I sold the Pegasus as I could not justify it), those “peaky”, few seconds, passages now sound perfectly fine in the context of the presentation.
With another Topping, namely the D90, there was a midrange harshness which I (and the other people attending the test sessions) found difficult to digest. Same with the Yggdrasil Unison A1, which emphasized the upper midrange and treble with too much enthusiasm (I understand the A2, now OG, is less V shaped than the A1, but V shaped nonetheless).
Still waiting to source a Pontus II or a Spring 3 to check if they are the step ups people are (more or less spontaneously …) raving about.