Every Fiio product I"ve had from the E07 to the E17 has been slightly dark. Nothing wrong with that, but I wish Fiio would move to a more neutral signature eventually. I think I agree with the statement that Fiios pair really well with slightly bright headphones that need a little bass emphasis.
I found the X3 by itself to be on the "tizzy" side driving orthos (a difficult load) but the E12 was completely neutral.
I am guessing this is more due to the X3 not having quite enough current for the orthos by itself.
On a related note, anyone have any idea what the battery life on the X3 is using LO only?