I guess I'll get to test this with my hiby r6 to see if you're right. If it's a damping issue it should be particularly apparent on sources with high output impedance. Usually the bass just gets bloated and boomy, but if you're right, the upper mids should be really noticeably shrill on my hiby as well.And I don't think it's a tuning issue. Its a sound damping issue.
Planars always struggle with damping, and this being a closed back iem, is hampered by that.
No way to get around that.
I might have to pull my fiio a5 from my sell pile until my shanling gets here....