There are common known issues on these amps with certain excessive heat generating components being mounted too closely to the main board (especially the two grid leak resistors; and this board did look toasty in that area); but for the most part here, I believe the issue was having the soldering iron too hot and/or holding too long in one spot, while replacing the caps and reflowing solder joints. Ultimately damaging the traces and solder pads, and then chasing their tales trying to repair them.