Just a quick question, can you expand on what you mean about the bit of your post that I put in bold? I don't fully understand what you mean there I don't think. "Coupling Insensitive", I think you mean headphones that don't change their measured frequency response much between people when measured with blocked ear canal mics,
Headphones like the HD800, Focal open HPs, some Hifiman open planars, would land in that category.
Blocked ear canal entrance measurements are a bit tricky to interpret properly past around 1kHz and very much so past around 5kHz (when presented the way Harman recently did it, in other contexts they can provide useful data higher), but probably the ones I'd prefer to see for now. Other types of in situ measurements have advantages but might also be trickier to perform well.
But in the context of that post I'd also exclude headphones that are consistent across individuals but don't follow the transfer curve between test rigs and real individuals other consistent, coupling insensitive headphones would follow (the DCA Noir could possibly be an example I believe). These headphones might actually be competently designed if that deviation is deliberate and ensures that they effectively deliver a good sound quality on most people's heads... or not.
or do you instead mean not much variation with different spatial reseats on a measurement rig (eg GRAS)?
I see spatial reseat consistency on a measurement rig, alongside pad compression and controlled leakage tests, as very useful proxies. It's not unreasonable to presume that a pair of headphones that has low positional variation, is insensitive to controlled leakage tests, and which SPL varies linearly with pad compression across a large part of the spectrum, is likely to also be consistent across individuals. The reverse however (ex : a pair of headphones that is very sensitive to leakage), might not be true (presuming that this pair will vary across individuals) as good design can mitigate these issues.
Since in situ measurements on a cohort of listeners are difficult to do, these proxy measurements might be the most practical way to get an idea of whether or not a pair of headphones has a chance to be insensitive to coupling issues.
And what do you mean by the error curve remaining similar to Harman's?
Since the OE HT has been so extensively tested via listening tests, and since for coupling insensitive headphones like, let's say, the HD800, we know that, once equalised to a target, this is what the test subjects actually experienced to a reasonable degree of exactitude, at least up to 4-5kHz, until comparable listening tests are performed with equalisation to other targets based on other test systems (ex : DF HRTF + tilt on 5128), I'd personally prefer to stick to what we know works. So if the error curve was very different for such headphones in that range, I'd have some apprehension about its relevance unless it's proven to be at least equally preferred.
Obviously we know that individuals vary in their preference to some degree around the HT, but you get the idea.
Headphones sensitive to coupling issues (in particular singling out the ones inconsistent across individuals here) may have a markedly different error curve, but that's most likely the fault of the headphones, not the target nor the HATS.