I finally fixed the hum today and wantd to share the good news. I replaced the wires to the output RCA jacks by using shielded cable from eyelets 14 and 7 on PC-5 to the right and left outputs, respectively. Also, the RCA jacks were grounded to the chassis and to eyelet 8. I removed the ground from eyelet 8 too.
Now, the hum is gone and sounds wonderful without that distracting hum.