Pros: Comfort, detail, soundstage/air, price, easily driven
Cons: Bass impact not for everyone
The AD900 is well known for its patent Audio Technica house sound; emphasizing light, airy highs. Separation of instruments is fantastic, but occasionally can cause the soundstage to sound artificially big. This is readily apparent when playing something like the Beatles’ stereo remasters, which have hard-panned left and right channels. While not common with more recent masters, this panning can be dizzying and fatiguing if the effect is strong or if one channel is favored. This is more a problem that stems in the studio, so I won’t hold it against the 900. Where the soundstage becomes a benefit is in live recordings; you can hear the musicians shift their weight, glasses clinking, low murmurs and clapping, all from their respective places.
The AD900 is great for acoustic guitar, strings, vocals, as well as jazz and instrumental records. They reveal a tremendous amount of detail and are best utilized with FLAC files or SACD (though I haven’t had the chance to try SACD for myself since my Panasonic player doesn’t support it). Percussion is brought out very well and feels realistic. I would, however, not use them for distorted rock or bass-prominent music. They are very accurate and balanced, which means that they will not give you the bass impact for genres like techno, house, dance, etc. Music with a lot of distortion or gain tends to be tiring to listen to (the highs can be rather shrill). Distorted rock music often doesn't reveal much detail, so I tend to favor the DT880 for that because the sound is so smooth and has more impact. I have an average sized head and these stay in place pretty well. The comfort is a 9/10 and the complaints about driver angle absolutely do not ring true for me. They are extremely light phones whose pressure is distributed evenly, for a comfortable listening experience. The cups don't provide a seal so expect to listen in a quiet place or turn up the volume. I haven't owned a great deal of headphones, but the 900 bests the Sennheiser HD555 and the DT880 in comfort (admittedly only slightly for the latter).
Watching movies with these is fun because the soundstage is so wide and involving. Dialog is articulated clearly and crisp. I would recommend these, but understand this headphones' strengths and weaknesses before buying, otherwise you will be sorely upset. You don't find too many used models online, but they're a good value anyway if you’re looking for a neutral headphone with lots of detail. One thing: packaging could be nicer; they come in a cardboard box with a plastic display and stand. It's fairly sturdy, but you're wondering "how could this be an audiophile headphone, packaged like that?" Thanks for reading, hope this helped!