Preamps can make a difference. Grace pres are very nice. Niant pres are too. I use a V3 and like it very much, these days usually via a SPDIF connection to the 680 when I need more than 6 channels. I find it sounds nicer than the 680 stock preamps, but I don't hesitate to use those. Usually the choice for me hinges on practical ones like much I want to carry, how many batteries I need, and how many channels I want to record. Generally I find microphones more influential, especially given the quite decent performance of the stock preamps in the DR-680. Personally I'd question mics first (and their setup but sticking with the gear question). YMMV of course, it does for others.
Maybe you've already been doing this, but if not.. run your microphones straight into the DR-680 direct for a while, play around with mic configurations unless you've already determined which setups you like best with the BSC1's in various scenarios, then if you can, borrow other mics and preamps to try and determine for yourself which way to go. Barring that, listen to other taper's recordings made with different combinations and weigh that against opinions. Your gear is already to the point where suggestions begin to veer into subjective opinion.. there's mine.