I can see that this question has been asked before, but I can't see that there was ever a clear answer, so I hope it's ok to ask again.

My CP4 distorts when played through headphones. When plugged into a mixer it's fine, but as soon as the volume is over halfway the sound in the headphones is distorted, especially on high notes. I have tried to adjust the gain, but it changes nothing - as soon as the resulting volume is appropriate for use, the sound distorts. I have of course tried a factory reset, but this makes no difference. I am using DT 990 250ohm pro earphones.

Can anybody suggest what might be the problem?