thanks for the advice guys, i bought some 580's today. from your advice, i decided to not bother with the 590's, and only compared the 580's with the 280's so i could see what less money and isolation might get me. listened to the 280's first, and thought they sounded very good. they were clear, and had a lot of slam, but they seemed to actually bend a few loud notes out of key... probably just being not broken in, but still disconcerting. At any rate, they sounded pretty good. But then I put on the 580's and it was just like "YESSSSS." Much better in all respects, and more comfortable, too.
The soundstage is CRAZY. I popped in my band's CD, which I co-produced, and edited and mixed myself. I was hearing things that are wrong with the mix that I never knew about. And I could clearly distinguish the signal from seperate microphones on the drum set. For instance, I could hear the hi-hat coming through its own mic and through the stereo overhead mics as seperate sound source locations. Through any other headphones or speakers (including my $3000 klipsch's) the hi-hat just sounds like one instrument coming from one location, but with the 580's on, you can hear the two signals seperately from each other. Which is a bad thing of course, but it just goes to show how important it is to have awesome equipment to mix on. Now I've got some awesome equipment.

Can't wait to break these babies in.
Oh yeah, and I'm not using an amp. While these headphones might benefit from an amp (haven't tried one yet), they are already the best I've ever heard even without one. I tested them in the store through the headphone out on their Sony component CD player. To get the same decent volume from the 280's and 580's, I set the levels to 3 and 5, respectively. Not that big of a difference, and certainly not like I had to drive the output into clipping or anything. I've got them plugged into my Pioneer receiver right now and it's plenty loud at 1/4 of max level. I might set it higher for short times when I really want to rock, but I'd be doing damage to my ears if I left it like that. Haven't tried them in a portable yet.