playing the fiio via LO extends it's battery to 16 horus, but why?, if it's supposed Line-out is well d'oh line-evel like 2v or closer to it, so volume is pumped at max, so by logic more volume, more power draw so less battery.
Beside what have been said (which the headphone amp disables itself when line-out is used), you need to understand what is actually happening on the headphone-out - it is "driving" the headphone, both voltage and current wise (*power = voltage x current). On line-out, because of the high input impedance of the portable amp, it almost doesn't suck up any current, and X3 itself turns into almost a pure voltage source. This means the line-out isn't actually "driving" the portable amp, and that saves a lot of power on the X3.
So the x3 isn't powerful enough on its own to drive headphones? It seems redundant to add an amp to something designed for portability. I wouldn't want an x3 unless it could drive most headphones on its own.
It is more than powerful, but there is always something better out there compared to the headphone amp section built into X3. Also, the definition of portability can be different from person to person.