I'm a Pro 900 owner and I've been searching for portable cans that were similar to the Pro 900 in terms of great SQ and heavy, punchy bass.
I was worried that I'd be disappointed with HFI-580's since I'm use to the Pro 900 as my main cans. I was also worried that the 580's wouldn't have enough bass. I considered myself a huge bass head and wouldn't even consider neutral headphones.
So I auditioned a few cans at B&H, specifically the HFI-580's and it's competitor, the M50's (which I had no interest in). I used my Clip+ w/E5 amp and the 580's sounded much better. As I recall someone saying earlier, the highs of the M50's were a bit too bright and slightly siblant. The sub-bass was not as clean as the HFI-580's and the mid-bass wasn't as punchy as the 580's.
I am extremely surprised by the amount of bass these had with my Clip+ and E5! 
I also auditioned the XB500. They had massive amounts of sub-bass but they seemed relatively muddy, too muddy for my tastes. I consider myself a huge bass head, and I prefer the bass of the HFI-580's to the XB500! 
I bought the 580's thinking there would be a chance I would have to return them, but after 5 minutes of listening to them at home, I decided they're a keeper.
I'm now a happy HFI-580 owner! I can't believe I worried that these wouldn't be bassy enough...well, with these 580's being driven by a simple E5 amp, they've satisfied my bass head cravings for sure! I think when I receive my ZO2 amp, I wouldn't even need the max bass settings.
I believe these are the best portable replacements for my Pro 900's. I'm going to be ordering the Pro700MK2 soon and I'm very interested to see how these compare first-hand.
*I keep seeing posts of people wanting to try dt250 pads, but haven't seen any feedback on how these sound with the 580...
Edited by MrNurse - 1/26/12 at 10:23am