Some will argue with this, but my experience is that X2 definitely benefits from an amp in general. (Depending on your source of course.)
Not as essential to have amp with it as it is with the high impedance Senns, but a good amp can improve bass extension, overall sound clarity, details, transparency, etc. even with a low impedance hp like X2. It sounds ok from an average source (not like HD600 imo), but far from its best.
I didn't find it satisfying/clear enough from my X3IIs hp out, so I used external amp. But now with my new DX80 I am impressed how the hp out sounds with X2. The amp I used with X3II doesn't improve the sound anymore. On the contrary.
I am still going to try a better amp withDX80 though.
This road never ends.