Pros: soundstage, instrument separation, bass quality
Cons: ear touches speaker = uncomfortable, bass quantity, back of concert hall
This is my first review so forgive me if I make any mistakes. Everything below will be in my humble opinion. I don't have good amp to drive them either, but most people agree that these cans do not need an amp to drive.
Supposedly, audio-technica's AD series is a line of very comfortable headphones that has warm sound. I didn't think they were particularly comfortable because I didn't like how my left ear is touching the driver on the left side of headphones. One possible explanation is I have weird ears, that might be the case but I have not had problems with other headphones before. Another explanation is the phones are old and the pads have been worn down. Anyway, this can usually be solved by inserting some sort of spacer(straws, tubes) in the pad. It didn't work for me, but other people has reported that it helped tremendously.
Another positive note to add is that the wing system works VERY WELL. It takes much of the pressure off your ear and you will feel very good wearing them for long time(if the ear touching driver does not bother you).
From little experience I have with UM3x, RP-21, and Mylarone x3i, I only thought the sound was mediocre.
Make no mistake, ad900 has bass and able to produce bass accurately above 40Hz. But it doesn't have enough bass impact most of the time. Sometimes, I can feel it somewhat but it makes me feel like I want more. It gives me a little taste of quality bass, but doesn't give the full blown smooth bass that I want. I don't think ad900 gives enough speed. It feels like it is always holding on to a note a tad too long. Maybe someone can appreciate this kind of bass and speed.
On the high and detail side, I am able to hear some new sound from ad900 that I have not heard from previous headphones. That is really wonderful for me. I really like that feeling that I am now really listening to music now by finding those additional details that I have been misting before. One of the complaints that made me want to sell the phones is the way vocal is presented. I always feel like the people singing are so so far away from me. Maybe this is what people call forwardness. To me, the sound is always there but not in my ears, if that analogy makes any sense. The violin sound is also far away from me, I want to sit in the front row to absorb all the details but I am sitting in the back rows. I can hear the music and see people playing but it just doesn't have the same impact that I want. Of course, if you like this laid back sound, try ad900 by all means.
Sound stage/instrument separation:
People say ad900 is very airy. If airy means everything sounds very distant, then yeah. Like I noted before, I feel like I am in the back of the concert hall.
Something AD900 does very well is instrument separation. I have a really good sense of direction of where the sound is coming from when I am wearing ad900. I don't know if this is contradictory to what I said before but it is true. I really like this aspect of the phones, being able to tell where the violins, where the singer, where the pianos are.
I hope my experience can help you guys decide if they want AD900 or not.
Excel in: instrument separation, very airy, able to hear the tiny fluctuation of the bass guitar strings.
Not so good in: ear touching driver = uncomfortable(YMMV), not enough bass impact, in the back of the concert hall.