Either it needs one of the output ICs, likely because this is the natural
stretch, or the DC offset is too much.

You change the ICs first, they are most common, or you can check them with a
scope with differential inputs. I think your odds are 70% or better that it is
the output ICs.

Do it right, clean the sink completely and put a BEAD of compound right between
the screw holes, DO NOT spread the compound ! This assures less air pockets.
These things are stereo power amplifiers turned ALL THE WAY UP ALL THE TIME !,
they NEED to get rid of heat and thus the installation needs to be done
correctly.

JURB