No Sound from Low Side Speaker on Panasonic Shelf Unit

[Nick B]

Hi all. I am a noobie with electronics. It's kind of a hobby for me and I'm good with fixing things electrical/mechanical, but just starting to get into the word of real electronics. Anyway I have a 1999 or 2000-vintage Panasonic SA-AK25 stereo shelf receiver. It's worked fine until recently it's started acting weird. A year ago I could tell when I opened the CD-changer the servo motors didn't seem to have the spunk they did (seemed weaker), same with the servo releases for the tape decks. It worked fine until just a few weeks ago after replacing the return spring for the CD-changer door. The left and right low-side channels (for the 6" speakers-there are 3" and a 6" speaker on each box) stopped working. The high side works fine for both channels (L & R), but not the low side. I can plug the low side leads (2 leads come out of each speaker box) into the high side and the 6" speakers work fine, there is sound coming out, but not on the low side. I opened the case and inspected everything as good as I could without taking anything apart. Looking at the service manual, I am mostly interested in the power amp PCB (has the transformer(s) diodes, big caps, transistors, resistors) as the speaker outputs run off it. The unit itself works fine- it powers on, the relay clicks, FL display comes up, CD changer works fine, radio works, and there's sound from the low side speaker outputs. Any ideas? Suggestions on where to start digging in and checking components, voltages? What should I test and how? I have a multimeter, soldering iron, and basic electrical tools. I attached a picture of the suspect boards. 

Thanks a million guys.