There are several issues.
1) With headphones connected the sound quality of the pre-amplifier output may be reduced (depends on the precise headphone(s).
2) If an amplifier is connected to the line out and switched off (to cut speaker output when using headphones) the sound quality of the headphone output may be reduced (depends on the precise amplifier used).
3) There is no independent control of volume or selection of headphone/line out.
