For what it's worth I wonder about some of the comments describing the X3 as overly warm, having a very noticeable mid-bass hump and the like.
The X3 to me is definitely more resolving than something like my rockboxed Sansa Fuze or Clip+ and there's a lot of detail and resolution on hand here.
One thing of note is also that there are still some issues with mp3 and ogg decoding which is why the sound quality should be judged by listening to lossless formats atm.
Fiio promised to address these issues in future FW revisions.
I think what a lot of people are describing as being overly warm in comparison to other players is due to the relatively high output power (for a portable player) that gives the lower frequencies more oomph and presence on a lot of IEM's and headphones as compared to players with lesser output stages that have less controll over the bass.
The sound signature is certainly not on the cold and overly neutral side but to my ears also not in the overly dark/somewhat veiled territory as some have described (not singling out anyone in particular).
Seems to me there's a bit of an expectation bias at play here since some of the early reviews described the X3 as being on the warm side.
I would describe the sound signature as well rounded and musical but not necessarily as dark or lacking in detail.
The X3 can sound quite bright with the right headphones, for example the Beyerdynamics DT990's. YMMV.
I've noticed this also. My X3 is more detailed and certainly not warm compared to my Clip+.