I've had the pleasure of owning the HD 598 for almost three weeks and I've almost no fault with the headphones. The HD 598 do everything well- a wonderful liquid-like midrange, a large, beautiful soundstage and smooth and absolutely unsibilant treble. (except for that peak from 17-20K, this can easily be fixed with EQ if you're sensitive to it though)
The only problems I have with it is simply the cable and the lack of sub-bass. The bass has definitely improved over burn-in, it's tighter and slightly extends more in my ears, however in bass heavy songs, I can't help but hear a lack of something. The bass is perhaps too overly polite in its presentation and I always find myself wanting just a little bit more punch.
I've heard the X1's have a beautiful soundstage and instrumental separation (though the soundstage is not as good as the Senn's) as well as thicker bass. (perhaps a little one-noted) The only problem I might have with it is the lack of replaceable parts and perhaps the midrange, which I've heard aren't too upfront with vocals.
I'd like to hear viewpoints from those who've tried either can (or both) and their own perspective on each of the phones. Any advice would be welcomed.