The main preamp before the power amp is in this section. Note the component locations which were left empty at the factory. These components can be assembled onto the PCA, and provide an additional 8dB of gain for the line out signals.

The line out signals exist at the 4 "zero" ohm resistors seen at the center of the image, above IC 302. Each one is marked with a small "O". The second image has the channels marked (yes the characters are tiny; you'll need to go 'huge', and maybe download the image to see them).

FL = Front Left

FR = Front Right

RL = Rear Left

RR = Rear Right

When soldering the outputs to these resistors, it doesn't matter which end of the resistor you pick (since they are zero ohms).