I've got a set of Fostex T50RP that I could cannibalize to build some DIY cans, but I'm intrigued with the AMT / FMT I've heard. The Martin Logan or Goldring speakers I've heard are pretty nice.
Those are cheap enough to play with. The size is small. For headphone volumes, perhaps they'd be full range enough to make decent cans? The low impedance seems like it might be the bigger issue, yeah?