A good SINAD number is a necessary but not sufficient indicator of good engineering.
en.wikipedia.org
Exactly my point.
Take the Topping L30 for instance. Great specs, great performance, good circuit design, good component choice and certainly not poor, but also not great, engineering and took a redesign to get it right. (like so many products b.t.w.)
The L30-II version .. no complaints about that one.
PA5... same thing.
IMHO such devices are released too soon and/or not tested all to rigorously.
That's the problem with engineering.... The circuit design can be great, component choice or practical application can be lacking.
To me great engineering not only performs (measures well) in all aspects (not just SINAD) but also is well built and durable with a low failure rate.
In all fairness... All of that is really hard to test for. All that can be done is measure it, test it to its limits. How it holds out in time is a lot harder to test.
And even then... something can be well engineered but a component manufacturer or the purchasing of parts may well slip and cause high failure rates even when the engineering was great.
That's how complicated real life is... this is not gauged in SINAD nor is SINAD an indicator of proper engineering and assembly. And this is what one buys. A product that measures well on a test bench.
What SINAD can show is good signal fidelity (of 1kHz sine wave) which is an indicator for good circuit design and suggests good engineering but it may still be lacking in proper engineering.