I can't seem to find the answer to this anywhere.
How do you switch between the headphone output and the pre amp output? Does one of the physical switches handle that or does it switch between them automatically depending on whether or not something is plugged into the headphone jack?
There is no physical switch. Headphones being plugged in control the function of the pre-outs. If headphones are plugged in, pre-outs are off. Headphones not plugged in, pre-outs on. So unplug headphones to listen to speakers.