The difference in DT770 and DT880 is not just the cups..
I know 880/990/770 share drivers, but there is some electronics attached to those drivers, which is doing things to the sound..
I also think that comparing them to the beyers is done solely because they look a bit like them..
Making an open phone is much much easier than making a closed one, you have much less that can go wrong in an open phone, smack in some good drivers and it will sound good, a closed phone is a completely different beast, because like the angel stated, you have sound bounching back and forth between the walls of the cup..
The thing with the restricted drivers, comming from working in a small room, I am not so sure about that, many drivers have a "blowhole" so that the pressure is not building up or goes down inside the cups..
Last I do not find open phones superior to closed ones, the closed ones can deliver a magic, which is impossible for an open phone to do, at least to my ears, also I like the added details you get fom a closed phone, they come forth because of the isolation..