I guess I could share my view on the X2's as I've had them for a few days now.
I've had the HD600's and I've been listening to them mostly just via Fiio E10 or simply the HP-out of my stereo amplifier. I was rather happy with it except that the bass was lacking a bit and sound was kind of boring with the exception of female vocals. And I listen quite a lot of music with female vocalists.
I found the X2's for pretty cheap (230$) so I thought I'll give them as I have low gear and simple amping possibilities.
First of all I was a bit disappointed as the vocals (female) just weren't quite as forward as with the HD600 and the bass was just really loud and sounded a bit "bloated". I feel like there was some certain frequency range in (possibly) mid-bass range which especially seems to overpower and give too much of thump. Also this could be because of the mixing of the songs. But the bass was loud anyways. Now that they have been burning in for maybe about 30 hours the bass has maybe eased up a bit but I feel the exaggerated thump is still there (in certain songs).
When listening to some live records the X2's were more fun to listen. I guess it is mostly the bass that makes these sound quite fun and exiting. So that was the main reason I thought I'll keep them and I also already found a buyer for my HD600's so I thought I don't want to cancel the deal anyways as I already promised to sell them. I guess I had too much trust for the X2's and all the reviews.
If you have some opinions about the Fiio E10 pairing with the X2's, I'll gladly hear them. I haven't AB'ed the X2's with the E10 and the stereo amp but I can't really notice a big difference. The weird thing with the stereo amp was that I had to turn the volume louder when listening to X2's than when listening to HD600's.
So now (that the HD600's are gone) the worst problem I still have with them is the bass. When I listen to music I tend to just slide volume loud enough so the vocals sound sweet and intimate enough. With the X2's this seems to result in my ear starting to hurt after like 15 minutes of listening even if I listen to rather bass light songs where the bass doesn't sound too loud (at least not by much) but sounds quite sweet and makes the song come alive pretty nicely. Anyone else having this kind of problem with headphone listening in general? I guess I'm just listening too loudly?
So for now I think I'll give them some more time to burn in and see how they'll sound. Then I guess I'll have to see if I have to buy back pair of HD600's or try HD650 instead