The screwed up midrange is mostly due to pleather phase doubling. It unevens the sound signature. It's not a simple EQ fix, because the phase is off. The focus pads don't have that problem and neither do the velours.
The 12khz bump is something you will see on almost every headphone, and indeed almost every microphone. I do a lot of professional audio recording for ****s and giggles and even my studio condenser microphones like the Rode NT1a have a 10khz hump. It's there in most headphones and most microphones because it makes music kind of pop in a good way. The problem is the recording already has the emphasis, and the playback through a headphone which also emphasizes it doesn't help. This is one of the things people mean when they say the 400's reveal flaws in the recording. If the guy who mastered the track used tricks like 10khz boosting and lots of multiband compression, it's going to scratch your eardrums out on the 400's.
In general, ignore the measured and reported frequency response of a headphone. Your ears, your environment, your gear, all make subtle differences in perceived loudness. Your ears don't weigh every frequency of sound the same. We are insensitive to some frequencies, and over sensitive to others.
A very good trick to use for that perfect headphone sound, is to use a sine wave sweeper program like Sinegen and spend a little time sweeping the frequency up and down, listening for perceived bumps or dips. Write down the frequency, and whether it was too loud, or too quiet, then you will know how to configure an equalizer to flatten it out a bit. The best part is even if you have weird shaped ear drums, or have partial deafness, it doesn't matter, it calibrates a frequency response TO YOUR HEAD. Works really well especially with something like Equalizer APO.
