I picked up one of these second hand last week http://www.qnktc.com/ab_11/AB_11.html . Currently listening to it powered off usb and through my O2, will eventually be the DAC I give to my wife for her desk. First I want to experiment with analogue filters on the DAC output and I'll also likely build a SE-SE 'The Wire' for the amplifier stage.
I really do have to stop dragging my other headphone amp/dac build out so long. Been a bit distracted lately by other projects.
Oh and I've had my AD900 cans re-cabled lately too! :)