One limitation of Studio One FREE is that you can only use one audio input (or two mono inputs) and one output with it.  So I've set up the complete audio routing and summing inside Scope.  The RME Fireface UCX audio interface is connected via ADAT with my Pulsar card and handles the analogue digital conversion of two Roland synthesizers. 

Thanks to CopperLan and ADAT I could get rid of Pulsar's breakout cable. That's why the output of Scope is routed back to the FireFace UCX over ADAT and from there to the active speakers.

This diagram hopefully helps to understand how my devices are connected among each other.

To be able to switch between the sub-mix of audio tracks that are part of the arrangement and the final mix-down track, I added the ControlRoom device.  More on this later.