Connect the speakers to the Line Out using a 3.5 mm stereo to 2xRCA cable and connect the headphones to the Phones or IEM output. Also, go into 'Setup -> Device Mode -> Mute Line' and select 'Toggle plugged' which cycles through the connected outputs (in the front) and the Line Out by long pushes of the volume knob. Thus, connecting your headphones and speakers the way I suggested above lets you change between headphones and speakers by a long push of the volume knob.

Also, when you disconnect a headphone after use, the unit will keep the output selected if the other front output is disconnected. Thus, changing headphones does not require to switch the unit back to outputting to one of the front outputs. However, when none of the front outputs are connected, the unit will not switch to outputting to the front.

If the latter is desired, instead of 'Toggle plugged' select 'Toggle Ph/Line'. However, this will not allow you to switch between the front outputs if both are connected and will output to both front outputs simultaneously.

EDIT. Also about the digital connection between PC and DAC, use whatever is most convenient to you.

EDIT2. As ramses says, the toggling function can be assigned to a button on the remote or a button on the DAC. To do this go into 'Setup -> Hardware/Diagnosis' and select 'Remap Keys' to either 'ON' (if you want to use the buttons on the DAC) or 'Remote' (if you only want to use the remote). Then scroll down and select 'Toggle Ph/Line' for the button of your choice.