Are you sure that you don't have pins 4 and 8 backwards?
I don't know how you could possibly get the power supply reverse polarized for that IC. The reverse polarity could cause the cap to fail but the Zener would have to open as well and that's not likely with the resistor that's in series with it.
Amp 1 must have an input to properly work. Your readings are normal.This is the reason you have a squeal and poor quality on sub channel.
If you aren't getting good audio from all channels, you may have bad RCA'S. You can check this with an ohmmeter. With out signal applied test the contacts from the back side of all RCA'S. They should all be a dead short.