Billheiser
Headphoneus Supremus
Everyone seems to point to the Fostex x Massdrop TH-X00 as a proper semi-closed upgrade to the X2. What it lacks in soundstage it makes up for in refinement. Personally, I don't see getting rid of my X2s even after upgrading to the TH-X00, but don't quote me on that as I've yet to get my pair but I love the cable options and open-back nature of the X2.
IMO, the X2 and the TH-X00 are on the same plane. I have both, and would have a hard time deciding which one to keep, if I had to chose. Would choose the X2 with my current amps; would keep the x00 if I made a big jump in DAC's/amps.