So I got ahold of SD and ask a few questions. Really about the AD>DA and looking to lock it to an external clock source.
i Kirk,
I got some answers from the head engineer.
By default, the outputs of the USBPre 2 only carry signal from the computer. There is a DIP switch that configures the USBPre 2 to send whatever signal is going to headphones to the outputs. This is how the USBPre 2 can be used as a preamp / converter in Stand-alone mode. When this option is engaged and the Monitor Mix knob is turned all the way to "PRE", the signal path from analog inputs to analog outputs is fully analog. So good thing I asked!
The USBPre 2 will lock its sample rate to that of incoming digital signal, regardless of what sample rate the USBPre 2 itself is set to (Via DIP switches in Stand-alone mode or via software in Interface mode). It does not do any sample rate conversion.
So no it will not accept an external spdif clock and allow mic input. But the is no resample on the analog outputs. Louie is right I'm all over this for a digi input to my 680