I have a few Class D amps I'd like to measure just as a learning exercise. My plan is to use REW to generate the stepped sine wave input and then capture the output signal as well, to get a THD+N measurement over the 20-20k bandwidth. I know this will require an attenuator because the output levels will be significantly above the sound card input limits. The amps are common ground and already have a filter at the output to reduce the PWM signal. Are there any other issues I should be aware of before proceeding?