Use line output. It ignores HIGH/LOW gain setting and outputs 4.5V p-p on full scale. The distortion and noise level is the same or slightly better than the headphone output at the same level (which is about Vol 119 with no load).
The main difference is that the line output is intended to be connected to a high input impedance device, like an amplifier. A quick measurement shows around 80 ohm output impedance on the line output, compared to < 1 ohm on the headphone output. It simply means you should never put any heavy loads on the line output. It's an excellent design by Fiio.