DDJ400 headphones strange problem

I bought a ddj 400 that I use with rekordbox and my macbook air M1 2020.

The volume of the headphones has a strange problem:                            I can turn up the volume until I reach about 25% of the "headphones level" knob travel, after which, the volume remains unchanged, it does not matter how much you turn up the volume.

I'm sure it's a software problem:
I tried with other ddj 400s and with my mac they all have this problem.   It is not a problem with the headphones because with all the other consoles they work (and the situation does not change even using low resistance cables)

however, rekordbox is updated to version 6.6.1
and the ddj 400 is updated to the latest firmware version available (9/03/2021)