The treble is awful for me as well, with the velour pads on. As soon as I put the pleathers on, the treble becomes acceptable and the bass really fills in. Have you tried pad rolling and/or modding? All of Hifiman's stuff is very responsive to small tweaks.
I'd try the "fuzzor" mod by Jerg, which will decrease backwave response, thus improving clarity, and tame the treble. While taming the backwaves is the primary focus of this mod, if you smush the foam less, you can absorb some of the treble. As a bonus, you don't need to open the headphones, it is fully reversible, and is really cheap.
I never found the mids to be distant on the HE-400, and I'm usually sensitive to that. That said, the Oppo's mids are so intimate, full, and lush. I think that you'll love them.