First off, the TH900 is a great headphone. The very first thought I had when I first put them on was-wow what an impressive soundstage! Nice and big, airy for a closed can. I agree that they don't sound like closed headphones really. I do disagree that they can compete with the HD800s soundstage-not only the size obviously, but with the pinpoint, laser-accurate imaging. I also feel that they are not quite as detailed as the HD800, but they are no slouch either. I wish the bass was toned down slightly, although my ZDSE does such a great job of bringing the vocals out on these that it's not such an issue anymore.
I really have primarily 2 reasons for selling these, and 2 alone:
1.) Soundstage/Imaging. I've grown accustomed, or rather addicted, to the hyper-imaging, holographic soundstage of my Eximus/ZDSE/HD800 rig. To the degree that almost anything else sounds 'wrong' to my ears. I know some people harp on the HD800 for being unnatural in that regard, but I feel I've truly dialed in my rig to a point that I've found my 'nirvana', now the 009 becons. :/ It took me a lot of amps, and sources to get this right-Iwas never really a fan of the HD800 until my ZDSE arrived and knocked it out of the park. The TH900 has probably the 2nd best soundtage I've heard, but the gap between it and the Senn is fairly large in my rig.
2.) Neutrality. No, the HD800 isn't absolutely perfect in this regard, but it gets pretty close in my system. I've grown accustomed to this neutrality, and now find myself addicted to the beauty of music that is close to dead neutral, if not quite. I've been a fan of colored headphones in the past (AD2000, Pro 2900, D2000), but I think I've grown out of that. The TH900 just doesn't have the flat frequency response that I've grown accustomed to-not that I expected it to. I do feel that it is the first 'fun' headphone I have heard that is relatively neutral and natural sounding to my ears, and if one is looking for a high end, relatively neutral but 'fun', detailed can, with an impressive soundstage-then look not further. The TH900 is definitely the best I've heard in those regards, and will probably hold that crownfor some time, I'd venture.
A word on the ZDSE/TH900 combo: WOW! I fully recommend this pairing with full confidence! The Zana is truly synergistic with the Fostex (as well as the HD800). The mids really bloom here, and pretty much make up for the (imo) sometimes overbearing bass of the TH900. Comparing the Zana (NOS tung sol/round plate/black glass)to the DP1 headout is night and day. I feel the DP1 is probably more neutral, but the EC really helps bring up the mids quite a bit-which is a must for the Fostex imo. Kinda weird though since my AD2K sounded nealy identical from both amps-maybe because of the already-forward mids of the AT. I will try my Brimar again tonight, and see how that fairs with the Fostex. So far I've preferred my Tung sol with every headphone I've tried-although it's not quite as nutral or dynamic as the Brimar. I'll post a few thought later on that...
I hope this is somewhat informative for those on the fence-and I think anyone can see that this is a purely subjective (mostly) opinion of mine.