Got my Senns HD598 a few weeks ago...BIG difference between that and my Senns HD428s. The 598s are opened cans so others can hear what you are listening too but because they are open, you get a larger soundstage. I can hear details I never heard before and it feels like 360 degrees of music around with wonderful separation. And they are also extremely comfortable.
I listen to primarily classical and jazz with some classic rock in the mix...and the 598s work great for all of those genres either from my iPod or My Mac computer. I do use amps but they sound great unamped as well. In other words, I am glad I upgraded. Just keep in mind the sound leakage. If that's no a problem, go for it. For closed sealed cans, I turn to my Sony zx700s. Hope this helps.