We wouldn't recommend using the line output for headphones.
Line level outputs typically have a source impedance of 50 to a few hundred ohms. They are intended to drive loads that have a high input impedance, usually greater than 5k ohms. They are optimized for very low noise and low distortion when used with high impedance loads.
Headphone outputs typically have a source impedance less than a hundred ohms. They are intended to drive the lower impedances presented by headphones which typically range from 16 ohms to 300 ohms. They are optimized for being able to drive higher current and for delivering reasonable distortion and noise with low impedance loads.
Line level outputs usually do poorly when driving headphones because their higher source impedance will not properly damp the driver at low frequencies and they can suffer from high frequency roll off when loaded with significant capacitance.
Headphone outputs typically have a source impedance less than a hundred ohms. They are intended to drive the lower impedances presented by headphones which typically range from 16 ohms to 300 ohms. They are optimized for being able to drive higher current and for delivering reasonable distortion and noise with low impedance loads.
Line level outputs usually do poorly when driving headphones because their higher source impedance will not properly damp the driver at low frequencies and they can suffer from high frequency roll off when loaded with significant capacitance.