The DX50 is $239, the AK100 (2nd version) is about $800 and the AK120 is about $1200. If sound quality between the three comes down to a "question of taste," the implication is that the difference in sound quality is ephemeral, and varies from person to person. That's why, after doing much reading about all three, listening to people on these threads talk about their likes/dislikes, doing Google searches for other reviews of all three, and thinking about did I REALLY want to spend $800/$1200 on a music player that I use to walk around with, I bought the DX50.
And, truthfully, it's not THAT much better than my $100 Colorfly C3 (I prefer it, sound-wise, much more timbre in the sound) but the C3 is more than adequate and was itself quite a step up from my iPhone 4/5S). UI-wise, it kicks the C3's butt, but is nowhere near as easy as the iPhone.
Maybe the crazy Pono will be the answer, with great sound and a wonderful UI. And Neil will convince all of the record companies to release their back catalogs in HD.
For me, the DX50 is easy to use, easy to carry (even attached to my C5 amp), and it delivers a great listening experience. And the combo costs about what an iPhone 5S 64gb costs, which is actually not bad.