Test conditions: 18V DC input, playing 1kHz sine wave from 16-bit WAV file into a 4Ω resistor.
Signal sampled by Focusrite Scarlett 2i2; using ARTA for analysis.
Here’s the spectrum produced at low output (0.5V RMS, 60mW):
At 2V RMS, 1W:
And at 10W:
Next: how to configure the amp.