I've seen Resolve talk about what happens with planar headphones when they break seal - the subbass disappears but then you get a higher peak in the bass further up the frequency range - that looks very similar to what you have going on there with the right channel. It's possible that the right earcup is not sealing properly to your rig (and maybe not to your ears either based on you saying you noticed an audible imbalance)......perhaps it's worth checking that there's not something "different" about how the pads look / fit / seal on the right earcup.I finally finished my measurement fixture and got to measuring my 400i. I felt like the left channel had more low-end compared to the right which wasn't very obvious all the time, but would sometimes annoy me, so I was anxious to see the differences.
-EQapo active with oratory's preset
-Put headphones on, measure, take off (repeat this 5 times and average the results for accuracy)
Here is the left vs the right response (12th smoothing):
And here are all 5 measurements I took of the left channel for those who are interested. They don't vary much, indicating good accuracy and repeatability (no smoothing).
TBH I'm not happy with this. There is an 8db imbalance at 20hz between the two channels. I bought a b-stock, so maybe the original owner knew this and returned it for that reason. I will see if I can get these replaced. If not, I can still eq the channels independently I guess...