Big fan of ASR and former HeadFi member (back when they actually had a sub-forum dedicated to measurements, data, and science). My only 'criticism' (and it's primarily directed at older reviews) would be Amir often using SINAD as a single metric to determine the quality of a product. I'd guess this was intentional, as a means to address how "convoluted" (aka batshit crazy) the hobby had/has(?) become, e.g. how you feel about your $1k audiophile switch is of no importance to anyone other than yourself.
Yeah, I think that's true. For DACs he doesn't even listen to them.
For DACs, I think SINAD (for those unfamiliar, this is equivalent to THD+N) really does tell something very close to the whole story. Call it 95%, 99%, 99.9%, whatever you will. The remaining N% is stuff we probably don't yet have reliable measurements for. I'm not saying it's magic and unmeasurable stuff, just that we don't have a solid set of measurements to capture it.
For headphone amps, or DAC+amp combos, there are definitely things not well represented by SINAD. I think most of this is related to power delivery (which he does measure) and I think the rest is probably related to slew rate or something thereabouts. But that's just a wild-ass psuedo scientific guess on my part.
Unfortunately, your wild guess does not fit experience.
Second partially true (as validated by intersubjective tests) guess is that the harmonic structure matters.
It is a major component of the coloration, and funnily enough low non-tapering one is discernible from low tapering one despite what SINAD/THD graphs suggest.
Amount is discernible too, especially of second and third harmonic, and total of higher harmonics, but we have not established the thresholds. And for lower harmonics, mostly at lower volumes unless it's a lot.
Intermodulating injected ultrasound medium to high level garbage (as produced by some poorly implemented power supplies in bad PSRR amps) is sometimes detectable and tests tend to miss it. It also sounds different from just bad SINAD/IMD.
Crossover distortion also sounds pretty bad especially at low volumes.
AES researchers (Earl Geddes, Lidia Lee, 2003) tried for a better perceptually weighted harmonic and intermodulation distortion metric in the past. It went nowhere because industry always wins. Even that is only a part of the answer.
And if the amplifier does outright clip, either in current or in voltage, that sounds just rough.
SINAD is a simple but good litmus test of DACs, if it has a bad SINAD it will be bad. If it has good SINAD it will be at least good. Given two devices with the same SINAD, one can be better than another in other measurements and/or listening tests.
Power delivery is a secondary parameter for qualitative analysis. Although for end user it may be actually the main parameter and SINAD is a secondary.
