The built-in amp is truly excellent! In fact, I have yet to find an external amp to beat it. I've auditioned several tube and solid state amps with the DA11, several in the $1000 + range, and none cold match the built-in amp. Of course, there are lots of headphone amps out there and I have only heard a fraction of them.
The DA11 would cost me about 2170USD.
What would I gain/lose if I brought the Audio-gd Ref-1 + Phoenix over the DA11? For me, the price would be 30% in favor of the latter.
I also got interested in the Decware CSP2 after reading a HD800 review done by skylab. Coupling the Decware CSP2 with something like the DacMagic or the Abrahamsen V6 DAC, what kind of sound would I end up with, still comparing it with the DA11? The CSP2+DacMagic combo would be only 70% of the DA11 price.
I realize that one strong point of the DA11 is it's humble size, to me it looks almost portable. (judged from a couple of flickr images where the DA11 is set up next to the Sonett and the Raptor respectively)