MG F/TF Awful Distortion

I have a Pioneer Avic X3 (Mofset 40 x 50) head unit connected to JBL

JBL GTO6428 rear and JBL GTO628 front speakers.



The speakers were new but neither have ever sounded satisfactory. They are generally ok to approx. half volume after which sound becomes very distorted. They don't deal with lots bass well, even at less than half volume.



I have tried another head unit and have similar issues....... Anyone have ideas what the problem might be?

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