I've just figured out my source selector. It's a 3 pole-4 position rotary switch.
The first two poles (A and B) are used for the 2 sound channels feeding the main amplifier. The 4 sources are:
1: Aux in
2: Line in (front panel)
3: DAC
4: DAC
3 and 4 are common - the DAC only has one output.
2 DAC positions are needed because the DAC has two inputs: coax and optical, which I'd like to select between. This is where pole C comes in. The switch below represents position 3 and 4 on the C pole:
The jumper on the left is the source selector on the DAC's PCB.
The LEDs show which source is being used. The 10k pull-down resistor selects COAX when the COAX LED is lit up. Otherwise pin SB is pulled high and OPT LED lights up.
It works perfectly.
I spent ages trying to figure this out, and it turns out that all it needed was a logic high or logic low on the SB pin...
edit: a photo of my current testing.