You will need to test each item in the signal chain - one by one - with different gear if necessary to try and isolate the issue to a specific piece of kit eg. input device / interconnecting cable / amp / input selected / channel ie. L or R / speaker cable / speaker / specific driver etc
Once you have done this you will be better placed to know which specific bit of kt needs attention.
And unless you are good at fixing electronics...chances are you will need to take it in to get repaired.
Don't know where you are - but the team at Axent Audio are great and fix a wide variety of gear for pretty reasonable prices.