Just saw this thread. Good to hear fireon's advice got it working for you.
You probably realize this, but I'd like to clarify for others that you can slave the DR680's clock to the clock of another coax digital-out device, and subsequently choose to record that input to the "extra" stereo channel or not. In this case, as will typically be the case for most tapers running an external preamp featuring a digital output, the intent is to record that digital input. But in other scenarios you can still slave to the external clock [DIN LOCK], and either record no stereo channel at all, or record a mix of the other input channels of the 680, without recording the external digital source on the 680. That can be useful when syncing clock with another digital-out recorder as a way of increasing channel count.
I've recorded 10 channels that way with the DR680 in combination with an R44. Most of the time in that scenario I have recorded two of the four additional R44 channels to the stereo channel on the 680 simply to make file alignment easier. Those channels are then duplicated between both recorders making it easier to align the files from both machines. And once initially aligned they remain aligned even over the course of a long recordings since they shared the same recordiong clock.