Cool.  Thanks.

You've got institutional access to that stuff or something?

Your whole body does affect it to a degree but it's mostly about you outer ear and how it reflects sound differently based on the angle that the sound comes from.  That's why it's shaped so funny.  Cancellations and reinforcements caused by its shape change the FR that makes it to your eardrums based on the the direction the sound comes from and no direction is ruler flat.  When combined with delays and FR differences between each ear it lets your brain tell what direction a sound comes from.  Everyone's ears fairly similar but none are exactly the same.

Those curves I posted are averages.  If you don't fit them that doesn't mean your hearing sucks.  The fact that different people have different HRTFs does not matter outside of the context of headphones because only headphones bypass it by piping sound straight into your ears.  Your brain learns your own HRTF and that is what you perceive as neutral because that's how real life sounds to you.  If your ears were mutilated and changed in shape then even RL wouldn't sound neutral to you for until your brain had a few weeks to readjust.  This has actually happened to people.

This is what it boils down to.  Two different people can be in perfect agreement about which speakers have the most neutral FR but can still disagree about what headphones do if their HRTFs are different enough.

These things can and have been measured.  If you test a bunch of people by putting tiny microphones in their ear canals and play sine wave sweeps from a dead flat speaker directly in front of them inside an anechoic chamber and average all of them you'll get an FR pretty close to a Lambda.  Once you add in room reflections the curves get even brighter.  Once again, remember that these are averages.  Also remember that it only accounts for sound heard at the eardrum and leaves out the tactile sensation of bass.  That's one of the tradeoffs I mentioned earlier.

If this doesn't help I'm not sure how else to explain it.  Maybe if you got to demo a Smyth Realiser and compared someone else's HRTF calibration to your own you could hear how they can effect the sound.

Most of the other stuff you seem to be talking about isn't even about FR and that's all that an HRTF deals with in this case.  I'm breaking these problems down into different parts.  A headphone with a perfect FR (however you define perfect) can still sound like crap if it has high levels of distortion and painful ringing.  The fact that all the Lambda models have nearly the same FR but no one thinks that they all sound the the same should indicate this as well.

There are good reason why your Mackies are still going to sound like your Mackies no matter where you put them.  The drivers aren't going to start ringing or suffering from cone breakup just because you put them in an untreated room.  They'll always sound more natural because they're out away from you and their sound waves can properly interact with your outer ear because they're not strapped to your head.  After a small distance away from a person's ears neutral is just flat.  If it plays a violin solo from the bottom of drain pipe it will pretty much sound like an actual violin solo at the bottom of a drain pipe because in either case the FR will be modified by the environment and then by your outer ear.  All those interactions with the outer ear get messed up and don't happen the same way if the source is too close to it.

You're right that a lot of headphones are shitty.  Many don't even attempt any kind of neutrality.  Different HRTFs don't excuse or explain that.  They don't excuse or explain painful ringing or exposed reflective plastic on the inside of a $1200 headphone.  They do explain a a degree of variation between the FRs of different models of headphones.

Why do I feel Beyer is using a random treble peak modeler to create their new headphones?

Maybe their engineers only ever listen to classical?

T5P is the only Beyer I actually sort of liked. TH900 is better, though.