I placed mine right at the output (wiper) of the volume control. I built mine from an LM380. I also placed a switchable 200 Hz wide CW filter between that point and my new audio amp. A throw of a toggle engages or disengages the new filter and a fixed resistor compensates for the gain difference so that it remains constant whether in or out. I opened R71 to save wasted energy. I am pleased with the results and cannot believe that I could improve upon it.
Your amplifier needs to have the output jacks isolated from ground since the output is bridged. Be very careful to do that. My only objection to it otherwise is the 10% distortion rating. Really? Is it actually that bad? Does it get into the rest of the radio, too? Might need some industrial-duty filtering. Let us know. The saved current drain and improved efficiency sure sounds tempting.
Don, ND6T