OK, I think I've found the culprits. On the 7-pin PCB header, which receives the three power xfmr secondary coils, the middle LV winding having a Center Tap, those three middle terminals showed evidence of Arcing on the header pins, as well as on the mating surface inside the Molex connector shell's single-wipe terminals.
Getting at the surfaces of the header pins is a PITA, as GK doesn't provide proper solder pads to unsolder/remove that header, so I had to resort to using first an X-Acto Knife blade, No. 15 blade, then an Erem tiny 45 deg flush cutters as a surface scraper, then an offset flush lead cutter (which leaves a straight lead 1/16" above PCB surface), also as a scraper, along with a tiny flat coffee stirring stick wrapped with #400 grit emery cloth, and finally a small dia brass rotary brush chucked into my Foredom hand grinder, now having gotten all of the burn marks off the pin surfaces.
Next, extracted the middle three contacts from the Molex Xfmr Secondary leads shell, scraped away the burn marks on the Molex terminals, then sanded that surface smooth, re-tensioned the terminals. Scraped off the burn marks on the inside surface of the nylon housing, washed that away with alcohol. Applied some Caig DeOxit onto the retensioned terminals and re-installed them back into the housing.
I then scraped away all the solder mask on the +/- terminals of the LV bridge rectifier pins (bottom side PCB), re-soldered those, then re-soldered the AC pins of the bridge on the component side. Also re-soldered the middle header pins, bottom side of PCB, so now there's solid connections from the LV winding thru the header to the bridge. I had also scraped away the solder mask on the other terminals of the bridge terminals, as well as those of F4 Pico Fuse, instead of leaving just the 0.080 dia solder pad on a 0.20" dia pad, masked off!
I've had other GK amps completely melt down with these connectors fused together. This one was on its' way, and I'm guessing the arcing eventually caused one of the 15A fuses to open. I never did find any failed semiconductors or other parts. So, trusting to luck and putting this back together to see if that's dumb idea or not.
On second thought, I'll first try powering up the LM317T/LM337T circuits. What happened to the LM317T when F4 fuse opened? No voltage thru the (+) side of the middle Bridge, while the upper xfmr secondary coils would still conduct. Would that have taken out the LM317T +15V Regulator, I wonder.