At the moment of writing this I am listening to my HD580s running from the E9 with the uDAC2 as the DAC/Preamp. It sounds extremely good and one of the main factors is that the E9 is the main amp. I ran the uDAC2 by itself with my 300 ohm Senns and didn't really have to put the volume past 11 o'clock but I felt that they weren't doing the 580s justice. When I got the E9 this thinking was vindicated. It turned these cans into something new!
One thing I have done that has also made a difference was to download J.River MC16 as my media player and set it to kernel streaming mode, therefore bypassing all that crappy Win 7 sound. This is direct digital streaming and it's wonderful!