Both LEDs flashing rapidly
Only seen when power is first applied and the amplifier is initialising

Both LEDs flashing slowly
The amplifier is scanning the inputs to see if there is a signal present. This state persists for 20 seconds and if no signal is detected the amplifier will revert to standby.

Red LED on (faint)
The amplifier is now in low power standby and will remain so until either a Bluetooth device is paired or (on certain models) a line input signal is detected.

Blue LED on
The amplifier is successfully paired with a bluetooth device. The amplifier will stay like this until the paired device is disconnected or goes out of range. 

Red LED on (bright)
The amplifier is active using the line input. When an input signal can no longer be detected the amplifier will revert back to an input check with both LEDs flashing slowly and if no other signal is detected it will revert to standby. If a Bluetooth device is paired whilst the line input is active the Bluetooth will become the active connection as Bluetooth always takes priority.