Some brief and, no doubt, premature impressions of the X5, which I've only had for two and a half days:
The X5's PCM1792A DAC implementation is extraordinary - its warm and organic Line Out is very appealing to my tastes, especially when used with amps and headphones that are neutral and transparent. The DAC's great sound, in combination with the fact that it is DSD-capable, has led me to abandon my original plan of using the X5 solely as a transport for S/PDIF Coaxial Out to an external DAC.
Prior to getting the X5, I had imagined using its Headphone Out only occasionally, but it's impressive as well, and I won't hesitate to use it when I want to keep my rig as small and light as possible, but... The amp section itself is on the warm side of neutral (which is fine for some headphones, certainly), but I can also hear a very slight veil of distortion - the X5 amp is not transparent (as best evidenced in the treble region, under scrutiny with the HD800), compared to using the X5 Line Out to the extremely neutral and transparent Meier Audio Corda Stepdance portable amp.
This distortion in the X5 amp section is so "fine-grained" that it doesn't really bother me at all for those times when I would want to maximize portability by using the X5's Headphone Out. I'm being seriously nit-picky here, but reading the data sheet for the LMH6643 op-amp, just now, it seems (this is just my conjecture) that FiiO chose the LMH6643, in part, for its relatively high current output (75mA) in combination with its ability to operate on supply voltages as low as 2.7V, with low power consumption. Texas Instruments doesn't market the LMH6643 for use in audio applications, but overall, FiiO did pretty well, thinking outside the box to chose that op-amp and make it sound as good as it does.
For my tastes, though (aside from the amp's almost undetectable distortion), when using non-neutral, "colored" headphones like the Sennheiser Amperior, the X5's Headphone Out (DAC+amp) is too warm and dark, but I bet it would sound great with a bright and bass-lacking headphone like the AKG K550. Though not at all bright nor lacking in bass energy, my OPPO PM-1 is neutral enough and efficient enough to sound really great on the X5's Headphone Out. In fact, and I mean this sincerely, the X5's bassy and warm Headphone Out makes the PM-1 (and thus the forthcoming PM-2) sound a lot like my Audeze LCD-2 rev.1 does with neutral DACs and amps - a good thing, in my opinion. When the less expensive PM-2 comes out, I can imagine a lot of people enjoying them plugged directly into the X5 Headphone Out - which, in my opinion, doesn't quite have enough power to drive the less efficient Sennheiser HD650 to their best performance.
Meanwhile, having tried a lot of combinations, including Coaxial Out to the DACs of a Beresford Bushmaster MkII, the CEntrance DACmini CX, and OPPO HA-1 - with various amps (including those just mentioned, plus my Schiit Vali, Stepdance and iBasso PB2 portable amps) and headphones (including the OPPO PM-1, Audeze LCD-2 rev. 1, and Sennheiser HD650), I again have to emphasize that I'm finding the X5's DAC section to be so appealing that it deserves to be heard via the Line Out.
Here's my favorite portable X5 chain so far, (having tried several headphones and rolling Muses 02, LME49990 and OPA1612 in my iBasso PB2 Pelican):
X5 Line Out > 15V-powered Meier Stepdance > OPPO PM-1
And for desktop use, which I haven't explored as thoroughly, the current winners are:
X5 Line Out > OPPO HA-1 balanced out > Sennheiser HD800 (but only for well-mastered recordings!)
X5 Line Out > OPPO HA-1 balanced out > OPPO PM-1 (for any and all recordings)
For reference sake, I'll add that, for the LCD-2 rev.1, I prefer the OPPO HA-1's own ESS9018 DAC, when used with the HA-1 amp, balanced out. The LCD-2 rev.1 has enough color on its own - it doesn't need the PCM1792A's warmth, in my opinion, but the X5's DAC is just exactly what the HD800 needs, with lots of clean power inserted. The OPPO HA-1's own ESS9018 implementation is too neutral for the HD800, but again, perfect for the LCD-2 rev.1.
OK, so much for keeping this brief, but I have to add, that just folder-browsing, I've still not seen any X5 bugs running the 2.0 firmware (which is not what I was expecting!), and I didn't have to reformat my cards within the X5 itself and load my WAV and DSD files afresh, although I think "x RELIC x" deserves some applause for patiently emphasizing that strategy for those who have had problems with their cards.
The X5 is a keeper!
Excellent review and impressions of X5! Thanks!
I have similar impressions with you, mostly on the X5 headphone output. I've used X5 for more than a month. Although I like the smooth sounding midrange of the X5 headphone output, I do feel it is lacking a little bit of air, especially for classical. Also lacking a little bit of micro details. For pop and modern genres, the tonality of the headphone output is pretty good, smooth and a touch warm. But for classical I prefer a more airy sound signature.
Again, you're right, the headphone output of X5 generally matches quite well with natural to natural-bright earphones and headphones. My DUNU DN-1000 sounds pretty good with X5 headphone output, slightly better than when paired with my DX90.
Will try more the line out with external amp later.