"pleasant sounding headphone? Warm tone, with some treble, and bass that's reaches a little low and is textured"- Yeah, that's about right to my ears. I wouldn't characterize them as cold and analytical as a DT880 or K701, SA5000 and certainly not HD800's surgical precision. But, going from my very vague memory of the Ultrasone PRO900's i'd definitely say the T90 stomps all over them in terms of separation and texture, so that'd make them more analytical in my book. Having a properly contolled bass response also helps a lot. I think that you find the HFI580 more "revealing" because you are accustomed to the hyper U-shape Ultrasone sound house. True detail is there when you hear more body and weight to the notes, without sensing a decrease in clarity, not just 'sharper' transients.
However, there are certainly more cold and analytical open heaphones out there, but usually the trade-off is average bass extention at best. I think you should try to upgrade to a better closed headphones if you're looking for a more refined HFI580, a Fostex TH600 maybe?
On second thought: No, I don't find the T90 as predominantly warm with some treble thrown-in... But they do possess some added warmness in the mid section that gives them a fuller and weighty tone than how purely analytical headphones would present.
Thanks so much for your response. I believe you perceptions may be right. I've been listening to the HFI 580 from some time now. Coming from them Vocals do sound more forward on my T90s. and there's certainly more weight to the notes. If the T90s have such a warmish tone to them then how come people still pair them with Tube amps like the Bottlehead crack? I would also appreciate any other info I can get on the T90 from other owners as well.