Been re-reading this thread as I work on my review.
Just wanted to say that while the sonic differences between the X1 and X2 are somewhat subtle, they're also very well targeted and the sonic upgrade is significant in my view as a result.
I also want to say that I'm getting quite a lesson with this comparison, the X2 measurements look worse in some ways, to my eyes. But to my ears it's quite a bit better.
I had an email convo with the engineer of the X2 and there are numerous internal differences with the X2 that according to him were subtle improvements towards their evolving understanding of what the headphone target curve should be. I guess my point here is that I'm learning that to get something right for headphones, you don't end up with something that's necessarily elegant looking. The ear, human hearing system, and the complications of coupling to it artificially with a headphone and trying to make it sound natural is horrifically difficult. As a result, the solution as seen in measurements is very complex.
Anyhow, it's been really interesting for me to look at the measurements of the two and see the X1 as having cleaner measured response, and then listening to the X2 with clearly better sound has messed with my mind....in a good way.
When things start getting close to sounding good, you have to be VERY knowledgable to have the measurements remain meaningful.
Funny thing is, it's not just the X1---the New P5 and Thinksound On1 don't measure particularly well, but they sound pretty good.
Anyway, the lesson for me is not only do measurements not tell you everything, but you have to be way more expert than you think you need to be to understand them.
Your ears remain the best judge.