Having paired the X1 with an E17 (with and without an E09k), a Creative SBZ, an X-Fi XtremeMusic, and a HRT Microstreamer, I will only conclude on thing regarding the difference between the DACs... they aren't really as substansial as I thought it would be.
In theory, the HRT Microstreamer should sound the best but it doesn't. The SBZ on the other hand is a touch warmer than the E17 and the XtremeMusic sounds thin and fairly neutral. The E17 to me sounded the most balanced, I particularly drop the bass to -2 and +2 on the treble through the EQ on the E17. This to me did eliminate a little bit of the bass bloat which was my biggest gripe (and it's also a minor one) I had with the X1, it's now at the level where I like it to be.
That being said, do I hear a huge difference between 4 different DACs? No. It's noticeable, but it's subtle enough that I couldn't care less and be too anal about it. Life is stressful enough already when the next thing I do not want to worry about is being too picky over something so small, just enjoy the music (and games, and movies).