I don't like Fiio. Their stuff is too good for the price :P
But seriously, I have an E5 and E7 and just bought an E9. The E5 is great for air travel with a good set of noise cancelling headphones and the E7 I use for my desk at work. To complement the E9, I got myself a pair of good $300 cans. Apparently everyone at the hi-fi site raves on the E9 as being pretty much by far the best headphone amp in its price range; anything better seems to cost 3-5 times more.
Back to the X3, from what it looks like its the E7 Wolfson DAC (the 8740 seems to be basically one of Wolfson's hi-end stereo amps) and E7 amp? If it has a built in headphone amp itself, does the x3 even need an addition external amp other than to maybe save battery (and use the line out instead of the headphone out). I note that the S:Flo2 has a somewhat noisy headphone amp considering the quality of the line out, so between saving the battery and using a much cleaner sounding E7 or E9, an external amp makes sense, but I wonder if the same is true for the X3.
However, if the E11 is a much stronger/cleaner amp, then connecting the lineout of the X3 to an E11 for an awesome portable amp experience is always an option. If anything, the quality of an amp always comes down to how clean the power/ground are and having the amp circuitry in a dif device than the mp4 sound processor never hurts (less devices on a ground chain is always better).
I just got the S:flo2 myself, but menu navigation of music on it downright sucks and the album art support is finicky. And for having such a beefy (relatively) processor (rockchip 2806), it seems pretty slow for even basic animations and scrolling. Such a pity that literally the best mp3 player available (at least for under 300, hi-fi man might be better at the high end) has such a dog-crap worthy interface. I wish I could revert back to my assembly/firmware programming days and fix it, but alas I was never that good anyway
Here's hoping the X3 is good in that regard too :P