If you don't mind my taking a shot -
The SSL 2+ gives you 2 mic/line/instrument inputs on a pair of combo jacks, as you know. This means all audio being sent from the 1680 will need to go into these two inputs. They are analog, so you can only feed them an analog signal.

Anytime you want to send audio from the 1680, you can send in a number of configurations - while always being limited to a total of two live inputs at the SSL 2+ interface.

From the 1680 you can send:

1- a single mono source - left, or right - recorded in Reaper as a mono signal to pan and effect with stereo fx later as needed.

2- two mono sources - panned hard left/right, or any blend from full "stereo" to mono (both centered)

3- a single stereo source - a stereo keyboard, drum machine, a stereo effect, etc. (with panning controlled at the 1680).

4- a submix of multiple source sounds - mixed and panned through the 1680 mixer controls - your 3 drum mics, for instance, but pre-mixed, which means your balance between kick and the other 2 mics (overheads, by chance?) must be carefully blended to taste as it goes into the SSL 2+ - pretty much as we used to always have to do with stereo or 4-track tape.

The only limitations will be your need for more simultaneous, discrete inputs and/or the need for direct digital input, which the SSL 2+ does not have.

You will likely set up the SSL 2+ inputs to unity gain (neither plus, nor minus levels) and control the gain going into the interface from the various mixer faders on the 1680 and before it, any external gear you plug into the 1680 inputs. The main idea being good gain staging, so that nothing that comes before something leads to unwanted clipping or distortion.

The analog outputs of the 1680 are sufficient to produce a good, clean signal with very little audible noise floor at unity settings (except at the mic inputs as you may know already, where it's almost always advisable to put a preamp in front of those inputs and keep the attenuator below 10 o'clock, if possible... lower still is better.

Hope this is a good start. There should really be no issue for you...