My point is that tonality-wise, which is the most important factor of the sound, there should be barely any difference between them. There are others, like time-domain accuracy which is about resonances and reflections between head and cup, and harmonic distortion, and some arguments that say none of them matter very much. Far less than the tonality, in any case.

By and large I have thought that all Hifiman magnetoplanars have largely similar sound. The technology is very similar, it's just a thin membrane on which conductive path snakes up and down between magnets laid out to create a tiny motor force, enough to move that membrane a little according to the input and make sound.

I have the Edition XS, which should be fairly similar to 400se. You see how nicely these curves hug each other on these other models as well: https://tpucdn.com/review/hifiman-edition-xs-planar-headphones/images/comparison-1.png

This is what I mean when I say these headsets should all have very similar sound.

I drive my Edition XS with their 18 ohm impedance using the $9 apple usb c 3.5 mm headset jack from PC. Lowish impedance of headset can be a little bit of a problem, not sure if this is a problem with Fiio K3. I can't see that stat on the product page. The Apple dongle is very cheap and pretty good, though.