Recently I bought the X3 2nd gen as a replacement for my iPod but soon after I discovered that using iTunes with the iPod always worked, and I took it for granted.
Volume Adjust, Library Index, MultiCD order, they all failed on me with the FiiO X3, sadly.
So I tried to solve every issue I encountered and managed to do so, my solution is called:
FiiOSync and is Mac (and of course iTunes) only.
It is a free application for the Mac which will sync your m4a and/or mp3 library from iTunes to the FiiO and while doing so apply the correct tags for ReplayGain track/album, Multi-CD track-numbers and title/album/genre copied from iTunes.
I figured probably more people would be interested in such an application, hope it will help some who struggled with the same issue.