I was using my Asus Xonar Phoebus (a sister card similar to the Asus Xonar Essence STX) to drive my 650's, and decided to try an external DAC and Amplifier to see if there was any noticeable improvement. Considered the Schiit Bifrost, but had also heard very positive things about the Asus Xonar Essence One, so I took one home to try out.
Well, there is most definitely an improvement to the sound quality. An external DAC is the way to go! There is virtually no audible noise (dependent on the recording), and the clarity is amazing.
The E1 drives my E650's easily. After some minor firmware updates, the sound quality is even better, and if you want to tweak the sound more you can roll some op amps. Decided to get a couple of op amps to try out (LM4562NA), and it really expanded the sound stage and provided even more clarity.
A bottlehead crack is on its way to me, so i'm going to see what the sound is like with the combo E1 and crack. Will update!