I've got an X5 and I love it to bits so far. Frankly, for my level of ear development and portability, it's perfectly excellent and I'd bet the X1 isn't too far behind it.
head-fi loves the O2 but others aren't quite so keen (go to changstar.com and you'll find a lot of counter-opinions on the JDS gear).
As far the JDS vs. the Fiio, I think the O2 might be the more popular choice but I don't know if it's better. A lot of folks like the
Leckerton UHA-6S.MKII as an AMP. If you're settled on the X1 as your DAP, I'd just get that and hear how it sounds with your 'phones and then, if you need more than it gives you natively, look for an amp and/or DAC to feed it to.