I don't see the comparison at all. Engagement-wise, it's no contest; the X2 are so much more 'involving' headphones. The treble can be grainy sometimes, but at least it's
there. Caveat, my HD650s were purchased used in 2010, so I don't know what the new ones sound like.
The HD650 pour music into your ears like a liquid. They
never offend except for the veil. The X2s provide so much more of a soundstage that I'm shocked they're so inexpensive. They also engage you in a way that neither my HD650 nor my HE-500 do. That said, I still prefer my HE-500s, which is
reasonable considering that they're not competing in the same consumer space at all and I love the little details. If I could have that planar sound with a better sound stage, I'd be very happy.
When my girlfriend and I were exploring music with her new X2s and my HD650s and HE-500, she was almost always more impressed with the sound stage and bass of the X2s than the liquidy sound of the HD650 or the precision and mouth-watering mids of the HE-500. Her only letdown was that they sound sorta crap from her iMac headphone port and transform dramatically with amping. In fact, I've never heard (in my limited experience) headphones scale that well with simple, inexpensive amping. I've heard that the HD650 scales well with amping, but I'm not willing to invest in an amp
just for that headphone. Heck, I don't even want to play with the settings on my Polaris long enough to properly hear the HD650 from it!
I got her the X2 because I heard that it's a headphone that can compete with the HD650 and it was at a price I could afford for Christmas (on sale). That has been pleasantly confirmed. Now all we need is a DAC and to hook my previous amp (original Schiit Magni) to her chain.