Hi,
I just bought a studio 68C this goes into an iMac Intel Core running Catilina 10.15 
Plugged my Fractal FM3 into in's 1-2 and my rack into in's 3-4. Linked my Presonus Eris E5 into main out L/R. Played first chord and...nothing !! no sound is coming through the monitors, vmeters showing the audio is coming in, but no sound. I tried recording with Logic and it works, I was able to record and playback what I recorded through the monitors, but still nothing can be heard from in's 1-2 3-4. 
Any other audio source from the Mac is played back through the MAIN's 
If I switch the Mon function on, then I can hear but it's coming out MONO ! 
I tried every configuration I could but still no success. 
Please come back asap, still having 5 days to return this back 
Thanks
Best Regards
Marco