No power supply problem here, but I do believe what I encountered is a flaw with the circuit design. A certain component will die in that positive signal section of the woofer amplifier after a decade of usage, causing R311 to lose connection and its resistance in circuit will no longer be 15K, but back to 120K (yes, it's a 120K resistor not 128K that I previously wrote, my bad...)
I don't know which component that died because I didn't have enough time to poke around to find out. The monitors are back with the owner. Maybe someday if another problem arise, lol. I think the big 6800uF 80V supply cap will need to be replaced soon considering its age.