I don't know where this obsession with uberpowered amplification for headphones comes from. 99% of headphones are, in reality, very efficient devices that require little power. The Xonar STX can swing twelve volts, which is far beyond any reasonable volume for an HD650, which outputs 103db at 1 Vrms. And the distortion and noise in the amp is far below what is introduced by the transducer itself.
Amps like the M3 are elegant DIY projects, you can choose a variety of gain setting for the listening sweet spot, however the same could be done by the STX, up to +18db. I use that setting for the HD800 and the card's amp can keep up with all the peaks. Or you can get a b22, which could be designed to power speakers. Regardless of the elaborate design and good measurements of separate headphone amps, the same function has been available on a single chip for a while now. It's like vacuum tubes -- nothing wrong with them per se, but you can get a transistor to do the same job with much greater efficiency, comparable or better specs, and a lower cost.