Well, agree with the first part - bought four of them just for drivers, made custom wooden shells etc. But one u70 remained, and I know what's the main problem with this - driver is totally closed and just needs some air - simple overdamping, there is even thick felt here already, two discs on each driver. Basically, few holes around perimeter, under aluminium cover, can make those sing. And new earpads, of course. Will post some pictures soon.
Hmm.... I also tried the U70 out with new pleather earpads and just left the aluminium backs off (stuck the drivers down with a bit of tack around the edges) and it didn't sound so good to me. More airy, yes... but nothing worthy of praise.
I also tried out some light damping with open backs, but no good came of it. I'm interested to see what you came up with - they're ridiculously easy to mod, so I might try something out inspired by your work.