The O2 amp is what i need to drive my 600 ohm headphones none of the Fiios go over 300.
Plus fantastic reviews from just about everyone who has heard them, especially the JDS build.
Just got my O2 from massdrop and put in the ODAC. Most noticeable improvements from my D10 Cobra are that it's slightly more quiet in the background and I can drive my HD600's a little louder without my ears hurting.
Seems we have a pretty diverse crowd of people wanting different portable amps...
On a side note, V-Moda's new amp should be coming out soon! Although it's still going to be in the ~$600 range...