So tonight I watched a full movie via the K13, previously I did not do this, just music and TV shows. Swapping between X9 and K13 revealed something interesting, in NOS mode on the K13 (with EQ set to Off) and full bypass mode on the X9 inc the DAC filters, the K13 seems to have a deeper handle of the low end FR in cinematic through my speakers with a noticeably (though not hugely) different stereo image presentation. Whilst the cinematic bass certainly lends better for movie watching (and gaming).
For stereo image it seems to be doing the mix decoding differently to the AKM Velvet DAC in the X9 which is resulting in this observation. Neither is better or worse than the other, it's just different, I think I prefer movies via the K13 as a result and I think this is due to the movies being naturally 5.1 or 7.1 and being down-mixed into a stereo stream via LAV decoder (example, tonight's movie was Atmos 5.1 eac3 source being mixed to 2.0 PCM for, same sample rate and same 32-bit float depth) in the player.
I guess the conclusion is that if you are not using any of the tuning options, DAC filters and so on and just relying on pure hardware, then K13 may be preferred as a DAC and pre-amp vs an AKM (and ESS really since I've had the DX5 II as well). I also think that R2R distortion lends itself extremely well with visual media like movies where the cinematics feel more authentic when accompanied by this kind of sound signature, like film grain in movies enhancing the experience on screen.
I did not notice the same differences when either was used as a headphones amp playing the same things, they still remain too close for one to win over the other outright if we are talking just headphones, and I don't think the X9's headphone output sounds £700 better than the K13 R2R still which is a positive nod to FiiO here given the price, though the X9's price covers all of its features which most amps costing even more don't even have, so people who do use those features will probably get better value out of that side of things.
Still using the same connections, USB (power isolated) from PC, XLR to power amp.