I own the AK100 hi-res player, and several others from Hifiman, Olympus, Roland, Zoom, and Tascam. This has allowed me to compare AK100 to those other hi-res players/recorders. None of them can compete with AK100 in terms of sound quality and usability. The Hifiman is the closest in sound quality (the Hifiman HM-601), the others sound limited, thin and weak compared to the AK100.
I use the AK100 with the Sennheiser HD650 and Hifiman HE-400. The Hifiman headphones is simply massive, a really good combination, but the HD650 is my go-to headset anyways due to acceptable sound and very good comfort.
Some 'audiophile' people on the net has said that the AK100 has a cold and lifeless sound. I wonder if those people has done any A/B testing with other similar hi-res players. I have and can confirm that AK100 sounds really excellent. It doesn't hype any frequencies as far as I can hear, it's really hi-res and neutral, and that is what I want..
There is one major drawback with the AK100: It doesn't do gapless playback. Too bad when listening to albums like Dark Side of the Moon or other concept albums.. The menu/OS is a little bit fiddly (touch screen) but better than all the others mentioned earlier except for perhaps the Olympus LS-3 (which has a limited, thin & weak sound). The price is high, let's hope that means that only quality components has been used.