What do the indicator lights on the front panel mean?

The A4TI has several indicator lights: 'On' lights when the amplifier is powered, 'Signal' lights at the presence of input audio signal, 'Limiter' lights when compression/limiter is protecting the amplifier, and three dB indicator lights that show output signal levels at approximately -20dB, -10dB, and -5dB from full power. The 'Protect' light indicates activation of protection circuits.