I got my 580s in yesterday after more than a month of waiting past the time I placed my order (to save cost on shipping / customs, I had it sent to my cousins in the US. . . who proceeded not to ship it to me, but to take it with them next time they came over the border). Anyway, here are my impressions after one day of intense usage, out of an Emu 0404 USB:
The first word that pops into mind when describing these is "natural". They are the most natural sounding headphones I have - sometimes, it's like I forget I'm listening to headphones at all. The sound is one that doesn't pop out at you to impress you, but one that kind of envelopes your head and you sink into the sound (I think that someone explained this well earlier in the thread).
I first noticed that instruments were more lush sounding than my HD595s, which I thought sounded a bit too thin for my liking. To use the word again, they sound very natural. The bass is a huge improvement from the 595s, and I think it's one of the best balanced bass I've heard in terms of its presence - it's not too little like certain other headphones (I'm looking in the 595's direction) but doesn't overpower to music at all, like my A900s can sometimes do. I think it's the perfect amount when it comes to having an all-rounder can - want any more and you're a basshead, want less and I'd be inclined to call you bass-shy (not that there's anything wrong with that).
The soundstage is also great, definitely better that the 595s, and different from my A900s. While the A900s still seem to have a slightly larger soundstage, the HD580s are really very nice in my regard and makes the music sound very (again) natural.
The comfort is quite nice, better in some ways than the 595s IMO. It doesn't have the "bump" inside the earcup like the 595s do, so your ear is less likely to feel cramped or touch anything inside. The pads also stick out more, sort of bowl-like, than most other headphones I've tried, which gives me a nice enveloped sensation (I know that sounds silly). It does clamp down way harder than the 595s, which was slightly uncomfortable at first, but I think it's loosened up a bit and will probably keep doing so. At the level it's at right now, I'm very comfortable with them, and I wear glasses, so they don't bother me in that regard. They're about as light as the 595, which is just one of the things I was looking for when I started thinking about replacing my A900s as my primary home headphones long ago (the A900s can wear your head down a bit). They don't seem to heat the ears up, but I've had the AC on lately so I can't tell how it would be like without it.
For burn-in, either it took me a while to get used to them, or they really improve after ~ 5 hours or so. I'm strongly leaning toward the burn-in theory because at one point I hated these headphones for sounding so flat and I don't think I could just pull such a 180 in so little time based on getting accustomed to them. I think I have about 12 or so hours on them by now.
For genres of music, they serve (for the most part) very well as great all-rounders. I like them even for rock, which some people say they don't do a good job at. Of course, they're not all that "fast", nothing like Grados for rock, but the richness of the instruments and good coherency, along with a perfect amount of bass, serves me quite well. The only genre they don't really do all that well is metal. I don't listen to much metal, so it's not a problem for me. They do sound best though with classical (WAY better than any other phone I've had, especially in terms of the dynamics) and jazz, and other instrumental music, especially when the instruments are given room to breath. Speaking of which, I really like the space that these headphones give to the area around the actual music, ny which I mean the darkness in between notes (I don't really know how to describe it, but it's really quite nice).
My Emu 0404 USB's internal amp seems to drive them well enough for me (of course, not having any other amp I don't know what I'm missing). It certainly sounds better than through the iPod, though even unamped it's no slouch (I mean, the great sound characteristics are still there, just not as defined and energetic as when amped). My 0404's volume know is set at 12:00 for my music, for anyone who's wondering. I also watched some TV and a movie with them, as well as played games, and they hold up well, not suffering in terms of a lack of volume or anything compared to other headphones. One curiosity to me, which I suppose comes with its high impedence, is that different mp3 albums that I've gained to equal levels (89 decibels) with MP3Gain sometimes sound louder or quiter than each other compared to the same albums on different phones. This isn't much of a problem - I've adapted by making some shortcut keys in Foobar to control volume so I don't have to keep reaching for my 0404's volume knob to turn something down, only to turn it back up again.
Anyway, so far I'm very happy with them (but then again, I had fairly high expectations to start with). The sound is very natural and quite rich, with a good amount of bass, and I can listen for long periods without any fatigue. These really are a nice value, and IMO easily beat out the 595s, at least via my equipment and for my tastes. These things really shine when you just lay back and let the music take you away.
