Pros: Powerful, crisp and clear - great soundstaging
Cons: Can become too bright with some phones
I write this as a regret selling it off... It really gave me the most thrilling headphone experiences I have had - together with my HD650's that is. With Beyer T1 it was too bright. But I think I am exceptionally sensitive to brightness. My friend loved the combo (m902 + T1), so don't take my word for it.
Anyway - be careful if you are sensitive to brightness in headphones, otherwise - this is great stuff. It's crisp and detailed and very much involving. With the HD650 - everything is great about it. Didn't use it as a preamp. The DAC seems OK - but I was not overwhelmed when a/b ing it to my budget CDP (which I also used as a transport)
In sum, all I have to say is: m902 + HD650 = fabulous