The Pro 900 is a borderline can in terms of drivability. It's acceptable out of an iPod, for example, but not optimal. It's much better out of my Cowon X7, and better yet with portable amplification.
The HFI series is not really ranked in terms of quality, and suit different purposes. The 580 is a bass monster, the 680 is practically as close to a flat and neutral can as you can get with an Ultrasone, and the 780 is somewhere in-between, though people do frequently report that they seem to have the best SQ of those. The 780 has the highest MSRP, but it's been going for relatively cheap for a number of months now. The 580 swings a lot in price. Both the HFI-780 and Pro 900 have worked fine for me as portables.
