I was passing through Akihabara today so I decided to check out some headphones I'm kind of interested in.
I think in general I prefer closed back headphones, although the fact that I was listening on a noisy shop floor meant that the opened back headphones fared particularly badly.
Fostex TH-600: Love the construction and the finish on the cups. Kind of reminded me of my Audio-Technica ATH-A900X but better balanced with more controlled bass. Then I listened to the TH-900 and after that the TH-600 sounded quite crude. Amazingly open, balanced and detailed. Great soundstaging. I forget, but I think someone on these forums might have said that if you are considering paying the amount for the TH-600, then you are probably already moving into cost no object territory and would therefore probably opt for the TH-900. Both were playing off the Fostex HP-A8 doing DAC and amp duties. I've heard it performing as a DAC before and enjoyed it feeding an amp and speakers.
Sennheiser HD-800: Loved the clean detailed sound. Fantastinc soundstaging, detail and separation, but something seemed wrong with the bass. I didn't get to try it out with a a range of music and I don't know how well run in they were, but I was surpised. Amp and DAC duties performed by the Sennheiser HDVD 800.
I was interested in a cheaper closed back headphone, possibly something with less warmth/bass than my ATH-A900X, so I had a quick listen to some AKG K550s. Love the appearance, but for the price (at least, the price in Japan) I wasn't impressed. Not as detailed as I'd hoped. I think I was looking for a ATH-A900X with less warmth and bass because the treble and midrange are gorgeous, but I think I'm going to have to spend much more to get the result that I want.
I also had a listen to a few Stax models, but didn't note which ones or what Stax amplification they were running off. Ugly cans, but a beautiful open sound. If I were a serious headphone listener/collecter I think I'd want some. I'd have to try them out wth a wider range of music, but I think that potentially I like them better than the Sennheiser HD 800.