Your subwoofer may make a loud thud sound when switching on/off from the power point when in auto/on mode.
While this "thud" is safe for your subwoofer, if you do want to turn off from the power point please switch subwoofer to "off" position to minimise this sound.
You can also keep your subwoofer in "auto" mode for when it's needed as this will sense the incoming signal.