The best I can give is a guestimation. I've not heard the Akg 700's, and when I had the Portaphile for review, I didn't plug into the hd650. Why not? Because the dt531 had a groovy kind of love with the Portaphile, that's why.
If the dt880 (which I found overly dry and unmusical paired with my Nad c542 and Corda ha-2 mk1 amp. I have a feeling I was a synergy victim) retains any of it's dt531 relation (and they are absolutely related in sound)... then they should pair sweetly with the Portaphile.