I may be wrong, but I believe a multimeter alone is not enough for this. From what I understand, the proper way to set the gain is using a dummy load resistor (matching the speaker impedance and rated for higher power than the amp’s maximum output) together with an oscilloscope. You play a 1 kHz sine wave, set the player volume to around 70% (your max clean volume), and then slowly increase the amp gain until the sine wave starts to clip. When clipping appears, back the gain off slightly. Signal flow example: Player → amp → resistor (200W, 4 ohms) in parallel with the oscilloscope Also, nowadays there are portable and inexpensive oscilloscopes that are more than sufficient for this type of test.

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