I'm delighted that you have written a quite detailed response and describing some reasonable points that I might have forgot or missed. For the sake of curiosity and also to strengthen our discussion can you show me some reputable studies or sources claiming that the Harman-target for headphones is somehow the more "correct" take on music reproduction? Not that I don't believe you or think it's bogus, I am just aware that among well-known studios and successful mixers/producers, many don't even know that much about Harman for starters, let alone using Harman-tuned headphones/speakers to mix and calibrate songs. So to broaden this idea I would love to see some examples.
I will admit that I don't have hard evidence about Harman being more or less correct. My idea based straight on the premise that the target curve arrived at for Harman was derived from a well-adapted room with top-of-line Revel speakers, thus would seem like something that would be considered a "good sounding setup" by many. In short, theoretically the result is akin to a mirror-image of a reference sound system but on headphones.
As
@markanini mentioned, it is considered a good practice to give a listen to multiple sound systems while mixing & mastering, thus Harman might not be on a hit list.
On the other hand, neither would be any FF or DF-voiced headphones, since these are both theoretical models. It is often discussed in gear reviews that modern pop doesn't sound right on tonally-correct gear and that makes sense to be true. Consider these mixes to sound good on cheap car sound systems (which have ridiculous bass issues) or on cheap radioes. Underscoring, neither of these would align with "proper audiophile" (aka non-Harman) gear either.
Also, read up about "house curves" for speaker EQ-ing - a declining bass-to-treble curve is considered something that typical sound system should be tweaked for to give a realistic room-sound impression. Looking at the
recent topic by @neutralguy it is shown that, at least to some extent, such a curve is basis of Harman's tuning.
I think your point about varied headphones tunings somehow could lead to a "falsified and disrespectful notion to the point of the artists" is a bit confusing. Like, do you mean that people using non-DF/Harman headphones are listening to the incorrect/impure "presentation" of the artist/mixer/etc...? Because that's like how 99% people listen to the stuff they like. And if you're talking about the mixers/engineers then its back to my previous point.
I think I might have came out disrespectful which was not my intention at all. I'm just raging against audiophile community going after everything but the music itself. Purposefully mangling the record's aesthetic just feels wrong. The 99% people using imperfect gear to just listen to music on anything - I don't have anything against. They might just be missing out on some aspects of the record.
That is my personal philosophical digression which honestly doesn't hold under scrutiny, since one might argue that mangled stuff might result in new aesthetics being born and so on... Consider this to be the worst part of what I put in words
. It's just frustrating that music doesn't seem to be the priority sometimes.
I'm unsure how this headphones is the correct presentation of the original intentions of artists. And even if it is, then what have we been hearing/mixing all the time before the existence of 371 (and the few previous Harman-tuned headphones)?
I do appreciate the sound quality it delivers and the objective selling points of it (FR, driver material, seal....), I also do respect the brainwork behind 371. I do not judge it just on "personal enjoyment" at all, however I am cynical on the objective claims of this headphones and I prefer to see crystal-clear evidence.
Amusingly to some extent the mentioned HD600 are close match to Harman, barring the bass shelf. Many highly-regarded headphones which came out back before 2012 seem to be Harman-compliant to a higher extent than can be incidental (SRH840). Might be that Harman guys just put the idea into proper research and done the work to formalize it.
I feel that since there have been many headphone voicings in the past rooted in esoterics and showroom aspects, I believe that there's not one pair that really sounds realistic to speakers (which is impossible anyways without active processing), which are the common mixing target. Harman is also a "sell more" curve, after all, but rooted in a general idea that seems to make sense. You could bring up binaruals mixed for headphones, but these make like, what, 0.1% of recordings?
I agree open vs closed is not a just comparison, however not all dips and peaks (especially when it is very narrow) are easily audible. I do not think the Beyers (both 880/990) or Sennheiser(6xx/600) is more "tonally correct" than 371 though, I said I like them more and I think they both are way more impressive than 371 - And this is indeed a personal, subjective opinion. I do not have enough expertise and appreciation in the industry to be bold enough to say whether something is truly "right" or not.
Lastly, do you have any recommendations for smooth-FR headphones? I would like to try more to broaden my experience, but as far as I am aware headphones tuning is typically very "wild west", every brand has their own ideas and they make tons of varied targets.
Let's just get something straightened out - I don't necessarily feel the K371s are the best on the market, they just lack major flaws which make many other products annoying, apart from well-known 3.5-4k dip which is bad and probably tells why I consider HD560s a much more consistent experience.
It might be that, after all, anything you consider better does sound more accurate due to your personal features. If there would be one thing I'd like to see adopted from Harman's research, it would be the insistence to keep the FR without wobbles, no matter what actual bass/mid/treble balance is adopted.