Mid-range?
I would dont waste money in it.
Go for the K1000 used. There is inclusive replacement for its drivers.
I believe he was speaking about that part of the frequency spectrum that resides above the lower & below the upper frequencies, but in a
way you're right ! Lower to Mid "priced" headphones normally don't have the ability to reproduce the musical information in this region in a
satisfying manner as they usually overemphasize the Bass or the Treble parts of the music
Hmmm... It seems you have a number of pretty good headphones already in place. You might want to consider getting a dedicated headphone amp that should give all your headphones the benefit of better support. All of those headphones are pretty good, the common thread to them not working for you may be simply what's in front of them !