DDJ400 - Headphone no bass

Hi there, I have had the DDJ400 for 10 months now, been working perfectly until today.

I've used the same headphones the entire time, however now my headphones is missing the bass.

My concern is that there is a hardware fault.

When I plug the headphones in 100% like I usually do, the headphones do not have any bass.

However, when I plug the headphones in about 75% the bass is now restored.

I've tried this with other headphones too, and the same thing happens.

Any suggestions on how to fix this if this is not a hardware issue?