I'm not proposing to use the full 2-hour test suite, for the vey reasons you mention. I'm proposing to use the
already standardized and widely used BS EN 50332-1 'Program Simulation Noise' test signal (described toward the bottom of
this page), which was originally devised to have spectral content representative of music and speech for testing audio devices. This test signal is
just 30 seconds long. As I said, SoundExpert's previous measurements have shown the null difference metric using this program simulation noise to be
consistently just ~1-2 dB higher (worse) than the same metric calculated using the 2-hour real music suite, over all devices tested (45 so far), and using different ADCs. This is strong evidence that the program simulation noise would make an
ideal analogue to worst-case real music (i.e. music that would produce the worst distortion through the DUT) to use in the null difference measurements instead of the 2-hour test suite. So that's just a 30-second recording and a few mouse clicks in the software to measure
all possible sound degradation of any part of the electronic audio reproduction chain. I can't think of a quicker, easier measurement with as much utility as that. Well worth a try at least I reckon. (The non-coherent distortion metric mentioned in the video I linked isn't directly related to null difference measurements by the way - I just mentioned it as it also uses music as the test signal, and to demonstrate that the standard multitone test is poorly correlated to sound quality.)
I see there are several owners of an APx555(B) in this thread - would any of you be willing to try out these null difference measurements using your AP analyzers? Preferably recording a device already measured here on ASR so correlations with SINAD can be made, but any data would be useful. A loopback null test would also be interesting to see the upper limit of the APx555 in terms of the null difference metric.