The 100W setting on the INPUT LEVEL switch is for use with 100-watt class tube amps. Even in this setting, the INPUT SIG/PEAK indicator (LED) on the WAZA Tube Amp Expander may light red. This can happen with certain high-gain tube amps, which may generate strong pulse signals outside the audible range that trigger the indicator and sometimes sound like a low-frequency hum.
If the indicator (red lamp) lights frequently, adjust the volume and tone controls on your tube amp and the RESONANCE-Z knob on the WAZA Tube Amp Expander to reduce the level.