Just to clarify, when recording to the 6 primary input channels of the DR-680 there is no input panning available. The only channel panning available is through the monitor (output) mix. You can set things up to record that monitor output mix to the stereo channel in addition to recording the direct individual inputs, and that is the only way to record a panned input channel. Even then you'd still have the unpanned input channels recorded as well.
Playing back files directly from the DR-680 through the monitor mix instead of as individual channel outs, you can do M/S to L/R matrixing on channel pairs, which provides width control but no panning.
You can probably set up the DR-680 to record the L/R output after it's internal M/S matrix. I've never done that, and like the other have suggested, it's usually better to record the independent M and S channels separately, but even in that case there is no pan control, only a width control.