if one DAP had it all, we would buy it and be done. I own too many DAPs when I really need 1, and really use 3 on a regular basis. my sony a15 lasts forever and is so nice to use you never actually think about the DAP. but it's 4ohm, the audio specs are meh(distortions), and the hiss audible on many of my IEMs. so I have to cherry pick the IEMs I use with it.
the clip+ is 1ohm, it has rockbox and I abuse the EQ possibilities, it's super small. really cool. but it does that buffering noise every 30secs or so and I end up also cherry picking my IEMs for something not too sensitive and not too low impedance (I feel like the soundstage/headstage collapses on the clip with really low impedance IEMs, can't prove it, only a small feeling).
the X1 measures better than both above, and it goes a little louder than the clip (the sony is clear last), so for many headphones, the X1 might drive them loud enough when the clip will let people wishing that it could go to 11. the impedance is 2ohm, while it's not 100% clear from audible changes with all IEMs, most of them(16ohm and above) will still have less than 1db change compared to a 0ohm source, so a lot less than 1db variation compared to the 1ohm of the clip. an in most cases they will sound about the same(maybe extreme stuff like a se846 will have an audible change in frequency response and be just a little warmer on the X1? IDK but it will measure that way).
now of course the X1 has a poor UI for a 2015 product, and that's really 80% of why I still have the other DAPs around.
in conclusion, most people can be very happy with all 3 of those, but a few will be very annoyed by one parameter, and for those guys the choice will be important. like me being obsessed with noise^_^, that makes the X1 the winner on sound for that reason alone, and the rest of the specs also show it as a winner. but it loses with EMI noises to both other DAPs. some will care, some won't.
that's how specific things can get when you're a maniac
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still in the 100$ zone, I believe the X1 to be the best fidelity choice there is ATM. FIIO did something good there.