Comparing the HE400SE and the Sundara re distortion I would say the HE400SE is better as distortion accross the board is low at 104dB & below, and you'd be listening to 104dB peaks with some substantial bass EQ at quite loud levels, that's the relevance of the 104dB line in the bass:
Whereas in the bass in the Sundara, which incidentally needs a lot more bass boost than the HE400SE, you have worse distortion in the bass:
The 104dB lines & 114dB lines don't have any relevance above 100Hz as you're not gonna be listening to 104dB/114dB at 200Hz and above, but you are quite likely to be listening to 104dB (and sometimes above in some circumstances) in the bass after EQ. So for all practical purposes the two headphones have the "the same" distortion above 200Hz as the listening levels will be low there, (below 94dB), but it's in the bass that matters in this instance between these two headphones, and the HE400SE wins out there by virtue of requiring less bass boost for a start, but by also having less distortion in that area too.
As for the frequency response between the HE400SE and the Sundara, the HE400SE is a bit smoother above 5kHz than the Sundara whilst both have good channel matching, and of course the HE400SE having the better bass extension as already mentioned - another win in my eyes for the HE400SE here, although I don't like the shenanigans at around 1800Hz in the HE400SE with that quick sudden dip, but you could EQ that area up with a fairly relaxed filter at 2kHz and you'd just be left with that small sharp dip (not perfect, but on balance I'd choose the HE400SE frequency response & it's a better value headphone too):
On a quick tangent, for some reason planar headphones often have these sharp/small variations above 1kHz, for instance my HE4XX after being measured on my miniDSP EARS and then converted (roughly I stress) to a GRAS measurement:
And then my dynamic driver headphone K702 measured on my miniDSP EARS and converted to GRAS:
You can see above that the dynamic driver headphone is smoother on a "micro" level.....just something I've noticed with planar headphones being rougher - the DCA Stealth is the exception though and they employed some clever acoustically 3D designed/shaped "baffle" to get rid of cancellations & standing waves, with the resultant frequency response being exceptionally smooth for a planar:
