1. The input sensitivity is probably equal for all input except phono, so it does not matter.
2, Assuming a windows PC it probably will automatically switch when it recognizes a USB audio device like a DAC. If not you can select the output manually in the sound and audio devices of the control panel (Audio tab). While you are there also disable the system sounds - you do not want that coming from your speakers.
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Asus 1002 & MediaMonkey > Musical Fidelity V-DAC (usb) > Primare A10 > Monitor Audio GR20