Good review but personally, I wouldn't use AKG 812 as a test HP for this DAP.
Cons: lacks sub-bass and weak in low bass into mid- bass, 2 dimensional sound with no sense of space...are indicators of the DAP HP amp couldn't drive the 812 properly.

the k812 has 35ohms impedance.  are you saying that the x5ii should only be used with iem's?

and the k812 comes with a 1/8th inch plug, which tells you something about what the designers think is required to drive it.  you need to use an adapter for a larger ho.  the k812 is designed to be easy to drive.

Numbers don't always tell the truth. For example, akg k553 is 32 ohms, 114 db/v but it takes more of the volume knob on my woo wa22(1.5 watt) than hd800 which is 300 ohms, 102 db/v. K812 is 36 ohms, 110 db/v. Imo, it will take a stronger amp to fully control the 812. If u have a good iem, maybe use it to better judge the x5. I had first gen x5 n wasn't impressed with it driving my hd800, akg but iem like shure846 was quite good.

"THOSE GREEN BARS DO NOT REPRESENT THE RATINGS I GAVE. THEY APPARENTLY CANNOT BE CHANGED WHEN I EDIT MY REVIEW."
On the mobile website, the bars do represent what you gave in the review, but on the desktop website, it's the average ratings from all of the reviews for the product.

@koiloco you're right. i should have also tried it with iem's.  i actually had intended to try iem's, but i wanted to move the device on to the next tour member and didn't take the time.  so re: x5ii with iem's- idk.

i added a "k812, no iem's" comment to my "cons" list.

fiio output 255mw @32ohms.
dx90 output 300mw @32ohms
k812 MAXIMUM input power 300mw@36ohms.
 
i'm dubious that the output power is enough to explain the huge difference in performance.

The AKG K812 even at 300mw and 36 Ohms will need a lot of driving. You would need at least to use High Gain mode and 50% volume. The sensitivity of the headphones won't be great when compared to IEM earphones. 
 
I don't know the maths however these would be comparable to the Sennheiser HD800 to drive. My AKG K702 on my X3 ii does need a lot of gain before they level out at a comfortable level also.

they are NOT comparable to hd800's. are you kidding?  i have hd800's and i could never drive them properly from my dx90.

The K812 needs 20-37 mW of power to reach 105 dB SPL according to Innerfidelity's measurements. The HD800 is an easy load to drive in comparison, needing only 6 mW; 2.0Vrms (Mid gain) should be just fine for the HD800 in that regard.

@miceblue thank you for your comment. it drove me to actually do something empirical.  i had been taken in by the hype about how hard the hd800 is to drive.  my dx90 can drive it just fine, with full detail, on medium gain, just as you said.  contrary to the numbers, however, listening to the k812's on the same medium gain, i had to lower the volume.  i went through my full list of test tracks, above, as well as part of a bach solo violin sonata.  i discovered that i loved the air/space/separation of the hd800, and thought them superior on every track EXCEPT the piano track.  the k812 has a sharper leading edge, sounding the note on the hammer strike as well as its persistent resonance.  the hd800 had a softer attack, and the piano lost some of its liveliness and delicacy as a result.  there's a passage about 2 or 2.5 minutes into the piano sonata where there are runs of high notes, sounding almost bell-like as they dance along.  at least they do with k812.  with the hd800, not so much.

This review squares almost exactly with my own impressions of the X5ii, and in direct comparison to the DX90 (in my case, running Rockbox firmware).  Bass reproduction is definitely lacking from the X5ii.  IMO, it sounds as if FiiO deliberately "voiced" the X5ii to appeal to "audiophiles" who often believe their hearing "more detail," when in fact all they're hearing is rolled off bass.  FWIW, my listening was done with T-Peos Altone 200, KZ ED9, and AKG K550 Pro.  Most amusingly, in comparison to FiiO's own X3ii, I found the X3ii to be a better (and more "complete") sounding player in just about every respect.  Make no mistake, the X3ii is not only the sweet spot in FiiO's current lineup, but their best sounding player. 

The speculation is that K812 have large magnets and with such a low power input on the speakers, you do need to induce a large current to make the sound from the bass to treble across the board. 
 
Another thing, these were designed with studio desks, high-power amps and other hi-fidelity kit in mind. AKG studio headphones "lack" bass in these sort of cans because of the reference quality they're aiming for. These are not aimed at the guy on the street looking to upgrade their Beats.
 
Typically I amp AKG K702 through my C-Que-8 Amp which has a large output. For me, they are not for enjoying music all of the time; I use them for reference. Then again, if you don't need the K812 I'll happily swap you .

@interpolate, want to explain why the k812, if indeed intended for studio desks, comes with a thin cord ending in a 1/8" connector?
if you look at akg's marketing video, here , at the end it shows people at a studio desk, but the first several images show people listening directly from pocket sized devices.

correction, 1 ipod, 1 computer ho, and then - i think- another computer ho, then the more professional uses.

so i feel a bit of an ingrate saying all this, but this is what i heard.
 
Grateful is not supposed to be part of a reviews conclusions.
 
You did Head-fi proud. No device pleases everyone.  That is eventually reflected in the review section.
 
FiiO e12 and X5 (1st gen) are not in any way "warm".  Having heard the X5ii now I'd say that continues though the highs sound better..or it's a placebo? They are not warm though which is fine by many.

The 1/8" connector is a just a standard that most manufacturers unless you go for the higher end Grado or models with balanced connectors which are available for the K812 as well.
 
AKG market to many of Apple users because they are naive and impressionable, not because they are over-rated basically it's a quick and easy sale. There is a reason why they belong on the AKG Professional section of the website. Like the AKG K3003 are in the consumer section as luxurious earphones however can be used in professional situations. Apple aren't as good as they make out but hey it sells products.

Finally it would wrong for everyone to like the same thing and some diverse opinions are needed.

This answers my question regarding the amping capability of the X5ii compared to the DX90.