At this point, I think you need to continue comparing the DC voltage across the resistors and the DC voltage on the various terminals of the transistors in the two channels. Where do you see a significant difference?
Sounds like you may have a cold solder joint on one of the smd resistors somewhere or a faulty one. Keep looking you will find it. Took me awhile to find the one on the punch 360a2.
Every Rockford amp I have worked on usually has a bad component when the bias does not work and its either a 1A or 2A or a 5D or A7 or there is a bad smd resistor.