Hmmm...
Well first I would like to see power ratings measured 20Hz to 20khz, rather than at 1kHz... (the 1khz measure seems to mostly be used for pro/commercial/pa amps?)
Typically, when you measure the same amp at 20-20k, you drop off about 25 to 30% of the power...
So in my head, I estimate that my Crown XLS2500, rated at 440W@8ohm (1khz, 0.5% THD) probably achieves a real 350W @8ohm 20-20khz, 0.01%THD.... However AmirM conveniently tested the 2nd generation version of this amp the XLS2502, and measured it at 310W @8ohm 20-20k, 75db SINAD / 0.018% THD+N (so my rule of thumb is not far off the mark)
My vintage 1980's Quad 606 is rated 140W@8ohm 0.01% THD (20-20k) ie: SINAD 80db
Both of these are excellent sounding amps... I really see no need for an amp specced higher than circa 0.05%THD or SINAD = 66db
Especially given that SOTA speakers struggle to achieve 0.5% THD (!)
So yeah - clearly the THD/SINAD can be used as one measure of engineering excellence (or even engineering hygiene)
But when it comes to the value equation... what is the worth to you of an additional 5db SINAD, above and beyond the threshold of audibility (which is somewhere around 60db / 0.1% THD)...
I picked up the Crown XLS2500 used for US$250.... (around 10 years ago)
The Quad 606 cost me about the same 30 or so years ago
Yes, the Benchmark AHD1 is a SOTA amp with very impressive specifications... but I seriously doubt it would be audibly different.
The value equation just isn't there for me.
In SINAD terms, I require my amps to provide 65db SINAD or more - and that is providing a 5db margin above the threshold of audibility .
Having said that, it is always worthwhile looking at the chart of thd to power.... 99% of the time, your amp is likely to be idling along at 1W and below... or for those who like it loud, in big rooms, 4W and below.... and with a heck of a lot of amps, the power rating provides almost a best case measure whereas the distortion produced at normal operating power levels is often substantially higher (for the Crown, 75db @ 310W, but only 66db @ 1W... and it gets worse with 60db @ 50mW
So when looked at this way, the Crown squeaks into the category I have self-defined, based on threshold of audibility.
I don't have an equivalent graph for the Quad.... I expect it would do better... on the other hand, in actual listening tests in my setup, I could not differentiate between them! (which sort of makes the point about threshold of audibility)
Also although SINAD is important, relatively high THD is inaudible, whereas noise at the same level, is highly audible.
So I prefer specs that break out distortion from noise... SINAD by blending the two, obfuscates the difference between them.