As for the difference... Let's just say that I won't be giving it away until I get a much better DAC.
A cheaper improvement would be getting an upgraded USB module from Burson.
I noticed more detail and instrument separation right away, I listen to a lot of classical.
and so far a bug free experience :)
I'm back to low gain and just turning the volume up a bit more. Reason is in medium or high gain, you loose a bit of quality and get a bit more noise and it sounds slightly more grainy. It's very subtle but I can notice it if I really pay attention to vocals.
I agree, Low Gain has cleaner sound. High Gain is somewhat foggy or blurred on my Senn HD800.