Hello, basically AUDIO ONLY bus powered USB is OK. However when you start adding LED's , small graphic screens & lit up buttons and such, it takes a toll on your USB output.

For a controller, the USB bus power has to do audio tasks as well as light up the various LED and VU meters.

Most large USB controllers draw up to 12V's and most USB's only dole out 6V so it doesn't make sense to run your controller on the "bare minimum"

At 1:46 on this video, keep an eye on the right side where the Play/Pause and CUE buttons are. You can notice them getting brighter after I plugged in the 12V adapter.