DJM S9 - Problem with knobs mixer (bass/filter)

Hello, On my DJM S9 I cannot properly use the righ-bass eq and the filter.

For instance, if I try to turn the bass all the way down to the low it won't cut propely. The sound will be cut off completely or either not cut off at all. In other words the knobs don't work. Same issue for the filter next to it.

It looks like the EQ and the filter does not work on this area.

This problem appears few months ago and very frustrating for a +$1500 mixer...

I tried everything possible (clean with supressed air on the area) , remove/replace knobs. Without success...

Please can you help??

Thank you.