it says "When you're using the USB input, the RCA jacks act as an output from the SDAC. If you disconnect the USB, you can connect analog input from your favorite DAC straight to the O2."
So I assume you want to use the RCA as output? Then you have to plug in USB and POWER and run you RCA out
When you remove the USB, then RCA is input, which it only acts as an amp, POWER have to be pluged in , in either scenario