Sacado del manual online:

Status indicator
Off: Subwoofer in standby mode or
disconnected from power source
Blue: Subwoofer on
With the Auto Standby function, the
SKW-591 automatically turns on when
an input signal is detected in standby
mode. When there’s no input signal for
a while, the SKW-591 automatically
enters standby mode.


Note
• The Auto Standby function turns the subwoofer on when the
input signal exceeds a certain level. If the Auto Standby function
does not work reliably, try slightly increasing or decreasing the
subwoofer output level on your receiver.
Resumiendo: sube el volumen de salida de subwoofer del recptor aunque para compensar bajes el potenciómetro del propio subwoofer. Esto hará que se dispare el Auto-On con mayor facilidad y con menos señal presente.