DJM-400 HEADPHONES OUTPUT POOR AUDIO

Hello,

I have a DJM-400 with audio issue on the headphones output:

-at low level (headphones level control close to -∞) sound is clear.

-at middle/high levels (headphones level control close to 0) sound is distorded.

Headphones device is OK & all others outputs work fine.

Could this problem due to failing headphones output operational amplifiers or DAC ?

Otherwise what can explains this issue ?

Sincerely.