Sorry to tell you, but this will never happen.
Fiio X3 got some potential.
I went from the S:flo 2 to a Cowon J3 and to this day (after a good six months using the J3) am bugged by how less resolving the J3 is compared to the old S:flo 2. Really hard to believe the difference. J3 is a wonder of a player regarding use and especially it's battery life ... but not even close to the sonic equal of the Sflo ...
S:flo 2 with an Arrow amp is/was by far, the best portable sound I've ever heard.
The Fiio looks interesting ... but I'm really much more interested in the iBasso DX50 ...
I rather enjoyed the headphone out of the S:flo 2 as well. Slightly better than the line out to be honest... markedly better sound stage I think. But the Arrow 3G's bass boost is like sonic butter to Shure SE535's and so always used the Arrow.
I might buy a used one if I can find it. Just don't think the battery is going to be good at this point, in just about all of them.
I really want a player with the dual DAC architecture. My desktop DAC (a MHDT Stockholm) is also a dual DAC architecture (R2R monolithic) and I like the way it renders audio as well.
As a S:flo 2 owner, when I saw iBasso had released its new audio player DX50, I was really excited and wanted to give it a shot. Fortunately, there is one headphone shop nearby selling it, I made a rather quick comparison there. First and the most important, S:flo 2 sounds better, I wasn't sure about it at first, but after spending a while switching between them, the result became very clear. DX50 sounds decent, but it sounds more compact than S:flo 2, it feels like everything is bound together when comparing with S:flo 2. The sound stage of S:flo 2 is noticeably bigger, instruments are much more separated, and the clarity of S:flo 2 really blows DX50 away. The earphone I used for comparison was Westone 3 with Shure Olive foam eartips, I only tried phone out on both devices since I didn't have an amplifier, the music files were 16 bits flac, I used microSD card on both players and listened to exactly same songs, the firmware on DX50 was 1.2.2, and 2.30 on S:flo 2, no EQ on DX50, and the EQ on S:flo 2 was set to "user" and flat.
My English is poor, hope you guys can understand me, I will try to write more about DX50 when I have some free time, I got to sleep now..