reihead...
Quite hard to say with volume matching issues (and that you cannot use the headphone and line out jacks at the same time it appears), however - with those caveats aside, the X5 headphone output has surprisingly nice extension at both ends of the spectrum, but sounds quite hollow in the mids compared to the line out / E12, losing a lot of 'emotion', seems to lack a little definition...
Must be said that that doesn't mean that the X5 headphone out is junk, not at all - infact, if your 'phones are very mid-centric, then you might actually be better off with the headphone out, however - if you crave lush[er], warm[er], comforting vocals / midrange from your X5, then the E12 definitely adds that for you...
That difference gets noticeably wider when going up the volume, the X5 itself not having as much control as the amp, which - to be fair is understandable (and on the most part, I wouldn't recommend being up as loud as I do on occasion - just recently, was listening to a jazz recording at maximum volume on the E12, the IE800s really open up the harder you push them!)
These though, should be said are still preliminary opinions, considering I've only had the player for four days...