I cannot compare the HD650 to the Opals as I never heard one pair, but the HD650 I own actually have this pristine clear characteristic that I love. I'm afraid to say that it's easier for me to mix with them than with my monitors since for the moment I have no treated room. I remember listening to the last Gojira album, and with them I could really feel the awesomeness of the mix, how easy all parts of the drumkit pierce the wall of sound, a bit "3D". I mean, compared to my monitor (yam HS80m) in my shitty room.
I'm not sure where do people hear "gentle highs". I'm not as pro as many here but that was not something that was too obvious to me, so ?
I can tell how easy they are to wear. For hours without any headache. They are light and confortable.
You should try them and the DT880 to find the closest to the Opals, but I still can say for the HD650 that, at least, you have one very good option.
I don't teach you something new about them with this post, but I wanted to say I own one pair of them and am really happy of them.
Just saw this thread is old but I have already written that post so I'm posting it.