Thank you for asking. I just want one that is very smooth. Voicing is an after thought for me, because I can change it. Bass efficiency is also important. The superb thd results of the apple product is the result. The Apple follows the harman curve till about 7khz right? In my experience most headphones start breakup towards the HF.
I don't really understand how you conclude from Crinacle's graph that the AirPods Max follow the Harman target up to 7kHz.
But I'd cast doubt that the K371, Audeze Sine, DCA closed X will reliably deliver it (or any target anyway) either, regardless of their measurements on a fixture. They're quite likely to have an
inconsistent response across individuals at lower frequencies for a start, something the AirPods Max's feedback mechanism
will be able to compensate for to some degree (and would do so even more effectively if it weren't for their non-sensical headband to cup attachment design).
In 2023 I'm tempted to think that most passive closed back headphones are sufficiently poorly designed and engineered that their inconsistencies (at low frequencies at the very least) are akin to blindly throwing stuff at the wall (the wall being here a cohort of listeners) and see what sticks (what sticks here being the few individuals for which the delivered response will just happen to match what they prefer). And it might not be that much better for active headphones with a feedback mechanism past the range where it operates either, if the variance is too important, or if the manufacturer failed to make a statistical analysis of where the range of responses effectively land on such a cohort of listeners vs an ear simulator. As a result I find it difficult to recommend anything of the kind to someone else.
To recap, regarding FR. Smoothness is our friend.
Smooth is not how I'd describe the AirPods Max past 5kHz. Actually, I'd describe it in that range as very un-smooth and unEQable, other than with broad filters to adjust the overall level.
But on the other hand they probably are one of the smoothest closed backs below 5kHz and ones of the easiest to EQ in that range... provided you actually know what the on-head response is
for you in the 800-5kHz range (above where the feedback mechanism operates).
Below 800Hz the combination of smooth FR + feedback mechanism ensures that they'll somewhat reliably deliver the same FR across individuals, and one that's easy to EQ to boot (and might not need it anyway). In that range, unless you get a really poor seal, you can EQ them with confidence based on ear simulator measurements.