There's many others saying it is just a mediocre player, producing mediocre sound quality.
Although the term, "Mediocre" wasn't often mentioned in reviews, other forms of that word was rather mentioned in comments/posts made by people.
Well, all I really want him to avoid here is "that" warm Fiio sound... The E11 was enough for me.
Honestly, I haven't read ANYONE even in your links say that the X3 is a "mediocre player, producing mediocre sound quality" as you say.
Are you sure you read Headphonia review carefully? It does not say that the Fiio X3 has mediocre sound quality at all, quite the contrary. They say the Fiio X3 "sounds closest to the E07K and is a sound signature that I enjoy very much" one that is "cleaner and more technically proficient than iDevices". They say it falls short in terms of resolution of some high-end DAPs like the Altmann Tera, Astell & Kern, Fostex, CypherLabs, Hifiman, VentureCraft all of which have a huge price difference with the X3.
The below link is detailed discussion of the X3 sound quality. See for yourself.
The Reddit link (see below) doesn't even talk about sound quality AT ALL.
The Reddit commenter seems to be an iDevice fanboy complaining about a few aspects of the X3 interface. Note that he concludes his rant by actually recommending the iPod or Android phone over "trash" like the X3. IMO, this last part should discount ALL of his comments. He's ranting about isolated reports of spotty 64 GB support on the X3 (without an sourcing of those reports), but then recommends people buy an iPod Touch which doesn't support ANY SD cards, or ANY lossless file formats. (???).
Head Fi user "bowei006 gives an extremely detailed and overwhelmingly positive review of the Fiio X3 calling it a "winner" (see link below).