Where to even begin with this ?
Perhaps there, where you'll learn that there is no such thing as a "flat at the speaker" (whatever that means to you) FR curve for headphones and that it isn't even desirable to begin with : https://www.headphonesty.com/2020/04/harman-target-curves-part-1/
What I was trying to say is a speaker with flat response. Without adding coloration to the sound it's trying to produce.
Interesting read there in the link you provided. I've been reading and playing - mainly around audio reproduction, home theater type stuff, car audio, etc. as an amateur. Headphones had advantages in my eyes until the advent of 5.1 systems came to fruition. Though, real nice floor standing speakers have been amazing to listen to as well with the right source material and amplifier.
Nothing in what was in that article had anything to say about speaker capabilities and "anything outside of human hearing is BS in a speaker stat". Here's a link to the Sennheiser HD 800S your article provided notice to. It's advantage is seemingly based in how the speakers are angled. Notice the sound capabilities of the speakers. It states 4 Hz to 51,000 Hz (-10 dB). Now, if that's not an important stat, why provide it for a $2,500 set of headphones? Just say - Hey - this conforms to the Hamon curve boys! And BAM - worth every bit of $2,500, right?
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Here's a link to McIntosh MC901. Here, they state the frequency response for both amplifiers. McIntosh is known world-wide as one of, if not the, best amplifier manufacturer in the world. Here, they say the frequency response of their amplifiers in this unit are capable of providing 10 Hz - 100,000 Hz -3 dB and 10 Hz - 70,000 Hz respectively. Why? Why is that important?

MC901 Dual Mono Amplifier
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It's important because it impacts what you can hear. THIS was my argument. You brought in to it a discussion about how the sound waves actually get to your ears, how the ear canal can impact the sound you perceive and how you may feel about what you are hearing. Very interesting stuff. I would argue BOTH are equally important. Perhaps that's why they call it engineering.