It’s kind of odd comparing a $300 headphone to a $1299 MSRP headphone (Bought for $540.) But here goes:
The bass on the TH-600 is better than the X2's without a doubt. It doesn't “destroy it”, but is a definite improvement upon it. The bass is a lot more controlled and hits fairly well. The TH-600’s bass makes the X2’s feel more boomy. The recession in the mids is very clear as soon as you test both side-by-side (aka the “v-shaped” signature). The high’s isn’t too overbearing either. I’ve read that they can cause fatigue, but I haven’t encountered this. Maybe it’s because it’s paired with the Tubemagic D1, which is a warm sounding amp. Overall, I like the sound signature of the TH-600. Something about it just sounds nice. Like others have said, it's also amazing how the TH-600 feel like an open can, even though they're closed.
I tested both cans with R&B, Pop, EDM, Jazz… My ears told me the TH-600 sounded better than the X2’s most of the time, but it wasn’t better by a whole lot. It was marginally better. The X2’s are really that good.
Physically, the TH-600 is superior to the X2’s. Cable is thicker and sturdier and has less microphonics. The X2’s cable makes noise when rubbed against other surfaces. It’s also a shame you can’t remove the cables. When you hold the two next to each other, the X2’s feel kind of cheap (Even though they are definitely not). Both are comfortable, but the TH-600 do get warm because of the leather pads. I do have to take breaks when using the TH-600’s.
In my opinion, the Fidelio x2’s give it a run for its money. For only $300 it’s amazing what they can do. TH-600’s are the better headphone, but not by much. I’m still weary on keeping them right now. I don’t know if I can justify the cost of these because of the X2’s.
Although, right now I am thinking of picking up the TH-900 or a LCD-2 with a new AMP/DAC to see if the differences are more dramatic. But that’s a whole different story...