I have owned this headphone for 2 years and it never disappointed me. The sound quality is no need to discuss as its sound signature has been discussed multiple times in TH900 thread. 
 
This beauty is really a beast, use proper amps with good ability to control (bass and mid), the TH900 truly sings. The first time I received it I can feel the sounds were great enough (I was using V200 or C4 to drive it). However, the bass was too strong and the treble was tend to be fatiguing. after 2 months break-in, the TH900 sounds more charming and the fatiguing treble got improved. But the bass still too strong to my taste and mid are a bit recessed.
 
I figured out it was my amp's problem and I changed my amp to the Luxman P1-u. Improvement? I will say it's HUGE. The bass got nicely controled and the Luxman bring up the mid nicely without losing speed, resolution. The treble sounds glorious, I would definetly recommend P1u as the right amp for TH900. But the Luxman only good in driving low impedance headphones, not big ponds like the HD800. (The Zana Deux - TH900 combo are also paires great to my taste.)
 
I'm not so good in describing sound but base on my personal experience, the TH900 is an extremely good headphone. I sure I will keep it as one my collection. 

I have heard it couple times from my friend. It impressed me very well. In the next couple days I received my HM901 but it has too many problem and the battery was end up broken, so I send it back and ask for an exchange to the AK120.

The sound is very balance. Bass/mid/tre sounds just alright, not too much and not less. Soundstage not as wise as HM901 but still great, depends on which headphones you use. So far I feel that the 901 with minibox card sounds a little bit better than the AK120 but definitely not day and night difference and in fact the sound quality of them are very close. Really depend on the sound signature people prefer.

With low impedance IEM I have to say it drive perfectly for almost all of them except IE800 and k3003 IMO.

Now I mainly use it to drive my UM Miracle with the DHC fusion cable. It should be my end game portable DAP! : )