These are my go-to set of headphones and I could have them on for 4-5h without fatigue or discomfort. I love the sound signature but understand it's not for everyone.
 
Highs: Clean, detailed, cymbals sound sparkly, but not fatiguing like the ATH-AD300
 
Mids: Well-defined, a bit recessed but fairly well balanced, warm vocals
 
Lows: Great bass (tight, punchy, accurate and do not bleed into the mids), no distortion like the Bose AE2 and QC25, not overly boosted like Beats
 
The only downside is live music sound like recorded music. In terms of spaciousness and wide sound stage, the LP2 is very good being closed-back, but it can't match open-back headphones like the AD series from Audio-Technica.Well, you can't expect a Ferrari to go 4x4 offroading, I guess.. ) So I ended up getting the ATH-AD900x for Live Music and Slow Rock.

Having a V-Moda Crossfade LP2 for a while now and very impressed with its sound quality and comfort as closed-back headphones, I was looking for a set of open-backs for certain genres of music that the LP2 does not deliver too well: live music and slow rock. For some reason live music just sound too much like studio recording and Bon Jovi's tracks like Always sounds too restricted.
 
After doing a bit of research, I came across the AD range from Audio Technica. I have had the opportunity to audition the AD300 and whilst I love the openness of these cans (compared to the LP2, especially for the above-mentioned genres), I couldn't listen to the AD300 for too long as the treble was fatiguing. I liked the sound signature though, so at least I was on the right track with the AD series. I then heard that AT recently launched the updated X series for the AD range and did more research on the AD700x and made a decision to purchase it, until I saw the AD900x for some $40 more and thought I'd get the upgrade instead.
 
The AD900x came with a rather plain jane box, a 1/4" adapter, a few pieces of paper describing the product and warranty, and..... that's it. (In contrast, the V-Moda Crossfade LP2 came with two cables, one that can be used to control music, an exo-skeleton case, two cable management pads for use with the case, and even the case requires you to cut a ribbon to open - it's almost like the difference you get in customer service at Hyundai vs. Mercedes). However, when I plugged them in and started listening (iPad + Fiio E1), I couldn't stop smiling for a while. They are very good, plain and simple.
 
Everything sounds spacious and airy as advertised. The highs are crisp and clear, but not fatiguing like the AD300. The mid-range is very nice, amazing for female vocals. The bass is there, precise and clean, but not as punchy as the LP2.  Impressive...
 
Playing live music through these headphones makes you feel like you are there. Bon Jovi and Axl Rose sounded like they now have all the stage space they need to rock. The AD900x is everything I was wishing for, to complement the V-Moda Crossfade LP2. They are both great, just different.
 
The two things that I think AT could improve would be to make the AD900x have replaceable cable like the AD1000x and AD2000x (come on AT, the 900x is not an entry level set!) and to use more Aluminium parts on the body instead of plastic to make it feel more premium. The V-Moda Crossfade LP2 has plenty of metal parts that makes the ATH-AD900x feel a bit cheap, or rather, less premium.

As a budget closed-back set of headphones, I think they are not bad. The treble is rolled off quite a bit, which is not great in regard to clarity, but good in reducing fatigue on long sessions. Unfortunately, everything else do not promote long listening sessions, e.g. ear pads make your ears feel hot and sweaty after a while (2h or so), the creaking noise every time you move is very annoying.