The practical answer is probably to do whatever is easiest because it won't make a lot of difference either way.
And this is why I think I'd prefer to use the 680 re: gang control via a single knob. Couldn't agree more!
And why it makes practical sense setting the gain of the V2 so that it's output is driving the DR-680 Line-input stage at a level where the 680's input trim for those channels is near the middle of its range, and also similar to the settings of the other channels running directly into the 680.
That will be a perfectly comfortable output level for the V2 (and input level for the DR-680) in a technical sense, but more importantly..
It's a practical thing for using the ganged function feature of the DR-680 to best effect. If the trim settings are all similar (total range is -31 to +31), you retain a wider available range of adjustment once you gang the input gain trims together.
The ganged adjustment range dead ends when any channel of the group reaches +31 or -31. Say gain trim for couple of your mics are set at +21, one at +11, one at -3, and your channels from the V2 are set at -26 (effectively doing a good bit of attenuation at the 680 input). If all those channels are then ganged together, the available range of adjustment becomes limited to +10/-6.