I am also using Hifiman Re-400's with my X3. The combination is certainly good. The X3 drives them well. I have had the X3 for just under two weeks now and only have about 10 hours on it so it may still need some time for it's sound to fully flesh out.
I did use the RE-400's with my previous player (Creative Zen) and I have to admit that so far I have to say that my impressions of the Hifiman buds are not as enthusiastic as many here and elsewhere. Initially I tried to tell myself that they are not light in the bass, but I'm sorry they are and significantly so. I don't want a bass heavy earphone but I certainly want one that provides impact in the lower register and so far the RE-400's just don't deliver in this area. I have the bass adjustment control of my X3 set to plus 5 and I'm still not getting acceptable output in the lower registers. I have experimented with various different tips for the Re-400's and I have managed to get a good fit with a very good seal so it's definitely not a matter of fit. I have owned several IEM's over the years and I think I have the know how to obtain a proper fit. Now to keep things in their proper context the previous earphones I used were Shure SE215's which have a reputation of being very bass heavy, yet I certainly did not find them to be so. Full and robust in the bass yes, but bass heavy, no. Monster Turbines are what I consider overly bass heavy earphones. I do also have some other issues regarding the 400's and it appears by the almost universal praise these ear buds have received that I may be the only one, so maybe I just don't appreciate their neutrality and simply prefer a more coloured sound, but enough about the 400's as this is a FiiO X3 thread. .......