ok... don't let the naming convention fool you. AD900X is dramatically different than AD900. i hate to use the cliche word of balance. but if you think of the frequency response curve of the AD900, which has a sparkly high and forward mid and very very shy low end, the AD900X pretty much bumped up the low, scaled back the forwardness, and scaled back the sibilantness as well. where i used to consider AD900 to be VERY bright and dry, and in your face. AD900X is what i would consider as "smooth" throughout the response scale with a hint of spark in the high end.
honestly, i tested it out with my HD598 at home, the bass of the AD900X has the same amount of bass authority, with proper roll off. of course i would always prefer less roll off, but might have to go planar for that.
most of all, the ultra wide and airiness feel of AT is back!!!. i miss that airiness soundstage. although, there's something to be said about the airiness that makes the soundstage (although nice), seem artificial, and a bit hazy as far as precision and imaging goes. the HD598's soundstage is less wide in comparison, but brings more depth and layers. i would say that's the biggest difference. with the AD900x, you'll get the well known reputation of ultra WIDE airy soundstage, as oppose to HD598, you're getting a lot of depth and layers and precision. if that still doesn't make sense. just think of AD900X's soundstage as a rectangle room that has a very Wide horizontal dimension, while the HD598 is vertical long hallway. but as far as instrument separation goes, HD598 still wins, it has a more precise separation.
so i wouldn't consider AD900X an upgrade from HD598, more of a side grade, or a different flavor. the wide and airiness is what i always love about AT's open design. :)
i am just glad they fixed everything i hated about the AD900. i returned the AD900 because of all the cons i listed above, which only made it great for a single classical genre but terrible for the rest. where as i would consider AD900X to be a good all rounder, almost.
oh and, thank god the driver no longer touches my ears like the old model used to do. i guess that has something to do with making it less sibilant.
feel free to ask me any questions, i'll do my best to answer. but so far, this can only require a few short hours of break-in to bring out its best.