I think you missed the perfectly valid critique, that sennheiser sub bass roll off very quickly. An eq filter won't fix this without distorting sound.
I hope you’re not thinking I’m advocating a crude N-band equalizer to broadly boost the bass and reduce the treble to fix the HD800’s thin-ness and brightness. I’m talking about very precise filters that the community has optimized mathematically to fit the measured response to the ideal Harman target curve: https://www.audiosciencereview.com/forum/index.php?threads/s...
There is no reason scientifically to believe this wouldn’t work, other than the maximum driver excursion and distortion limits of the headphones. Fortunately the HD800S bass is extremely clean and undistorted, and they respond really well here. I’ve tried this with other headphones, and most cheaper headphones do struggle greatly, while the HD800S sub-bass is extremely impressive, clean, and powerful when EQ’ed in this way. And even more importantly, this works wonders to smooth out peaks in the treble that sound too harsh to be prior to EQ.
Even if it couldn’t be fixed with precise EQ though, this is still something we can measure objectively via max SPL and distortion vs frequency tests. For example, another highly regarded headphone is the Focal Clear, and it unfortunately has severe bass SPL / driver excursion limits that rules it out from being corrected in this way.
Of course it’s a fair criticism that such expensive headphones should require EQ at all! And the HD800S are indeed criticized in this way on ASR! Sadly though, it seems virtually all headphones do require EQ to sound remotely near their maximum possible sound quality potential, in terms of their physical capability. I personally love the HD800S for a number of reasons, but one of them is extreme comfort (which definitely is subjective), but I will be the first to admit their sound is painful to my ears until corrected with EQ.
(1) We have an objective science that disambiguates and understands differences that are subjective versus universal preferences in speakers (to some reasonable threshold of population universality; e.g. maybe you find some 0.0000001% population that prefers horrible screeching noises added to any audio content, but arguing for this as an important dimension of subjective variability that we need to incorporate into hardware is I hope clearly absurd).
(2) The vast majority of subjective variability relates to differences in frequency response, which are extremely easy and cheap to adjust in software -- no need to cycle through many multi-thousand dollar headphones or speakers just in hopes of finding one that suits your preferred frequency response target.
(3) Almost every kind of imperfection that can't be corrected via frequency response tuning (e.g. undistorted SPL capability, THD, IMD, off-axis smoothness and similarity to on-axis for speakers, power handling and dynamic compression, etc.) turns out to be almost universally disliked versus when that imperfection does not exist.
(4) Once frequency response balance is factored out either via custom EQ or a "mass market" target curve (which for headphones is indeed more of a compromise, but for speakers is not even remotely a subjective matter, since there are mountains of scientific evidence over decades proving over and over again that the preferred anechoic frequency response for speakers is flat, i.e. the mathematically perfect identity function), which naturally yields a preferred in-room target curve of a smoothly declining slope -- very little (if any) subjective variability remains (e.g. bass boost or treble tilt to taste is one such example).
(5) To be clear, some subjective variability does remain, but it mostly relates to head fit and comfort (for headphones), and for speakers is much more nuanced and tends to relate to beam width and height differences. In both cases, the science is capable of examining these precisely, not in some kind of vague or hand-wavy woo woo sense. Lastly, there is always some need to make some coarse adjustments to bass and treble when listening to older recordings or recordings which were mastered to a non-neutral reference curve (as they should be, else it's impossible to ever reconstruct what was intended) -- but this is not so much a matter of listener preference as it is correcting for mixing/mastering errors in older or lower quality tracks.