I made a comparison between iPhone 6 and FiiO X3 when it was just released. I grabbed my customs - although very sensitive, not as impedance-dependent as TF10 - and thought the iPhone 6 sounds extremely tight and spacious, much more engaging than the X3.
You could argue the Apple is darker and more direct, but it doesn't cut off the treble. Considering the UI and ease of use, for me, I would prefer the iPhone for daily use.
However, in the meantime, I also got my hands on a friend's 6 Plus and the sound was muffled to me. Not as crisp and tight as the normal 6. My friend couldn't tell me which quality his files were, so I hope it's because of that.
But now reading about some more issues about sound cutting off, I am a bit worried the 6 Plus might be subpar.
I want to order the 6 Plus 128 GB soon. It would be great to have some more detailed impressions and comparisons.