Here is a challenge for you guys - I open a thread in the Sound Science forum with a link to download five music clips that was recorded using different X5 firmwares. Try and see if you can tell them apart. Answer will be revealed in a week.
http://www.head-fi.org/t/730848/fiio-x5-firmwares-sound-quality
Have fun!
BTW, I added an extra music clip for the comparison. Those who downloaded the original 5 clips should give the 6th a listen as well, and see how it compared. Cheer.