I'll add some comments on the HD-600 vs. X2 and give you my opinion. I've been listening to the HD-600, or HD-650 as my primary headphone for the past 4 years. The first 2 years were with the 650, but once I heard the 600 at a meet, I switched, preferring it's more extended treble, and lack of hump in the bass. I like both headphones a lot, they are both great in their own ways. 3 years back while still on the 650, I built a Bottlehead Crack with Speedball. Suffice to say, the Crack and the HD-6X0 is just a magical pairing. Even with crap $10 tubes, it just sounds fantastic.
A few months back, I got an Oppo HA-1, and started running the 600 via balanced cable on the Oppo. This just gives you choices. The 600 on the Oppo, incredible detail and resolution, crystal clear sound, gobs of power on tap. The Crack, more liquid, more bass boom (depends on tubes), super smooth. Is one better than the other? Nope. Just personal preference. I still listen to both. However, I had always been looking for a headphone with a bit more sub bass extension, and a bit more bass in general (think Harman curve). Also, my time for warming up the Crack or HA-1 for a listen is limited, and I recently started using a Ibasso DX50 DAP. With that in mind, I'm looking for a more plug and play headphone, but without losing the qualities of the 600. Enter X2. Sounds great on my Galaxy S4, Better on the DX50, even Better on the Oppo. It has that nice bass I was looking for, and the big surprise is that sound stage is so good, much more out of head experience. In getting to the X2, I tried LCD-2 (sold), PM-1(short demo) and PM-2 (send back to Oppo), but I ended up not keeping those because to my tastes, the 600 was better at everything but the bass extension. There was no value proposition there Spending so much money to get a little bass extension.
With all that said, is the X2 better than the 600 on Crack? Again, sound wise, that's a personal preference. I agree with Tyll, they are quite close. Once you move away from the Crack amp, the X2 is way more flexible, as the 600 really does need an amp, and to my ear does sound the best paired with the Crack OTL.
All that said, I'm just using the X2 now, and listening more because I can use them with my phone, or the DAP, or for really concentrated listening on the Oppo. The 600 just does not compete with it in the flexibility category. Still the 600 on the Crack is a special combo, but honestly, it's not getting much use these days. X2 is now my headphone.