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.
I know that this player costs only two hundred bucks, which should preclude it form any seriously lofty expectations, but I still have to say that I have mixed feelings about the FiiO's performance overall so far. It does sound better than my old Zen no question, but surprisingly the gap is much smaller than I would have liked or expected even at this price level. Maybe this performance anomaly should not be as much of a surprise as it is, as the Zen was a fairly highly regarded MP3 player, sound quality wise in it's day, plus the bulk of the files I listened to on the Zen, and the X3 as well of course, are Hi-rez uncompressed Wav files which the Zen accepted. Maybe it's just a matter of break in but I have never owned a piece of electronics who's sound changed that dramatically even with considerable break in time, speakers excluded of course. It would seem that I will need something of much higher quality and unfortunately much higher cost to meet my expectations and that sadly is just not in the budget right now. I have heard others make similar claims about the sound quality of other high-end DAP's such as the AK100. In the interest of full disclosure I have not heard any of the other so-called hi-end DAP's for comparison. I only have my Zen and my daughters i-Pod touch 4th Gen. for comparisons.
My Zen was in the beginning stages of giving up the ghost so i needed a replacement in any event and the X3 is certainly a very good product. In it's price range I'm sure it's in the top five sound quality wise of portable music playback devices, I just don't feel, at this juncture at least, that it's the giant killer so many of us had hoped, incorrectly and unfairly probably, it would be. I have none of the issues with it's user interface that many here have expressed and it certainly appears to be more reliable than my old Zen, time will tell difinitively if this bears out, so I will continue to use and enjoy it, at least until the X5 hits the shelves.