The HD598 has a larger and more spacious soundstage than the 600 or 650. I see you have the HD558 which should be comparable in size to the 598. I hear the soundstage as larger on the 598 than on the X2.
Funny how we each hear different things with the same HP's. I just did a quick A/B of the X2's and HD598's with some Norah Jones, Faith Hill, Jewel, and Corinne Bailey Rae. Soundstage is definitely better on the 598 vs X2 across the board, but the richer mid's on the X2 made Norah and Faith sound better to my ears overall. If you could combine the soundstage and clarity of the 598 with the otherwise fine mid/bass of the X2, that would be my personal holy-grail of HP's. Suggestions on HP's fitting that bill greatly appreciated! :)