Yeah this requires some background regarding how I view that stuff. Now… this is just my perspective on the matter, but I really think SINAD has the potential to be particularly misleading.
What you see on SINAD is just an index for one parameter that doesn’t necessarily have anything to do with sound quality past a certain point. In short, it’s useful to indicate if something is a flawed design, but beyond that it’s kind of meaningless since incremental gains are way beyond the audible threshold anyway.
Now to be clear, I’m not saying the data isn’t useful, but rather the representation of it in that index will often get taken the wrong way, and people are bound to make purchase decisions off of completely meaningless information. In my view, there are all kinds of additional things that contribute to ‘good sound’ than just this score, and I think this is also quite obvious when you have the opportunity to test the well-scoring equipment against the less well-scoring stuff.
For example, looking at an extreme example, a Magni 3 Heresy might do better on that index than something like a Bakoon Amp-13R (I can’t confirm this, but just imagine it’s the case), but when you listen to them side by side and volume matched with a high performance planar like the Susvara (I did do this test, and it did get loud enough on the heresy), it’s a night and day difference. You’d also reliably get this conclusion in a blind test. Similarly, the Rebel amp sounds noticeably better than the A90 to me (again, volume matched), even though the A90 measures incredibly well.
So, there’s way more to the story of what contributes to good sound than just scoring well on SINAD, and in my experience you can end up with some truly bad sounding setups if that’s all you go by. It’s no secret that I’m really not a fan of the A90, even though it measures well. To me it sounds sterile and mushy - and I know I’m not alone in that opinion.
To put it another way, in my view, scoring highly on SINAD is like saying “headphone A is better than headphone B because A goes to 50khz and B only goes to 30khz”. Human hearing at best only goes up to 20khz… and even then most people cap out considerably below that. Moreover, the more salient question about sound quality is less about how high up a transducer can produce sound, but rather the sound pressure level for various frequencies along the way. Another analogy would be like when gaming mice would all be marketed as having 8000+ DPI… when nobody playing competitively/professionally would be using anything above 1200 (there may be some exceptions here but you get the point).
TL:DR - as long as it’s not a broken or borked design, which would be quite obvious, you don’t have to pay much attention to SINAD.
Now with the question about scaling and the HD6XX vs the HD560s… I really think this is only something you can understand when you get the chance to hear it. If you can, go to a local shop and try an HD650 or HD600 off different equipment and see how different things sound when comparing an HD560s off those same sources. There’s a reason why some people consider the HD600 to be an endgame headphone off of certain types of equipment.