| An unbalanced line output is intended to deliver low power to a relatively high impedance load (> 1 kOhm).
If you try to drive headphones with such an output, it may not be able to deliver sufficient voltage and/or current to adequately drive the headphones' lower impedance.
Determining a priori whether the latter is the case requires knowing the relevant specs of the amplifier and headphones. Since the source impedance and VI limits of a line level output are rarely specified, you may need to just try it. However, if one has the appropriate test equipment, some dummy loads, and the requisite knowledge, one can characterize the line level output and answer your question.
Note that, if you drive the left and right side of the headphones (paralleled) from a single mono source, the headphones' load impedance will be half of what its manufacturer specifies. | 
