I received my X1 today.
The smell and the sight when you first open the box is really something, lol. Really smells like a new car. And those pads are literally sparkling, I swear for a second I thought they were covered in diamond dust
I've tried it on for about 40 minutes. At first the clamp is a bit tight, but it loosens pretty quickly. The thing is HUGE on my head!
I don't find the X1 to be the bass monster some have depicted. There's warmth and rumble in the bass, but there's less midbass than in your typical Sennheiser, and I'd like a bit more of that. The X1 is accurate, fast, musical. Not even a hint of sibilance. Good vocals, good punch, good energy and crunch in the guitars. Seems well suited to many genres.
The X1 doesn't have the crazy imaging and separation of the HD650, or the sizzle of the HD558. There's soundstage, but the sound comes forward as a whole, not in separated layers like with the HD650. There's detail, but not so much as to make this an analytical headphone. "Musical" seems an appropriate word to describe the X1.
As I said, I'd appreciate a touch more midbass, and I'd also prefer a little more shimmer and sparkle in the highs. However, while the X1 will never be a problem for those who are sensitive to high frequencies, I find it more energetic and less tamed than the HD650. I need to spend a lot more time with the X1, but upon first impression, it really makes me think I may have found something to replace my ATH-AD2000.
Oh, one final note: the X1 leaks sound like nobody's business
No wonder the box explicitely says "for indoor use", lol.