REAPER can send/receive control messages via MIDI-- it's probably a matter of some "simple" settings.
REAPER's Action List is where you'll assign physical 1680 control movements to REAPER functions. See the REAPER User Guide for the procedure. After you've finished, be sure to export your Action List as a KeyMap. Who knows? Others may want to do something similar.
My gut says make the 1680 "Master", but my gut has been known to have bad days...
I no longer own a 1680, but there are some features I miss which were not included in the 2000 or 2480.