I agree. But as long as this DAC is used with volume control why worry about how it performs at 0dB or at -90dB as long as it performs good in between?
I would be interested to see THD+N at say -60dB, -40dB and -20dB as that is where this device will mostly be used. Don't you agree?
The reality is that the distortions here are unlikely to be audible. But I am somewhat esthetically bothered by engineering misses when it wouldn't have cost any more to get it right from end to end.
THD+N vs level at several frequencies is an interesting and often useful measurement. But again, we have to be ready to partition the results into D and N, as well as understand the nature of each.
I also am an advocate of multitone measurements- in my reviews, I use a 42 tone test signal.