schmalgausenThe USB port provides power and data for the DAC portion. The external power brick provides power for the amp portion. Plugging the external power brick in will power on the amplifier and switch the RCA jacks from a pre-amp output from the DAC to a line input for the amp.
USB and RCA only: DAC is active, and RCA is set as output (amp is unpowered)
Power brick and RCA only: DAC is bypassed and RCA is set as input for amp (DAC is unpowered)
Power brick, USB, and RCA: DAC is active, RCA is set as additional input for amp