Help me hack my Yamaha Receiver
For what its worth, your orange dot is definitely NOT the correct place to inject a signal. It's inside the feedback loop, and the circuit is going to fight you.

The correct place is the pins identified at the left edge of the schematic, for example, "FL". These are the input to the differential pair that is the first stage of each output channel.



EDIT: Actually, I'm not sure what those "POE" pins are for. The correct way to apply the input signal is between the "FL" pin and the "star ground" line that you have identified with your green dot.