When UC is running when you launch Studio One, it is operating with the mixer page open. You will need to disable the mixer so the DAW controls the audio I/O. You can do this several ways:
- Assign Stereo Output channels to a Cue Mix
- Manually bypass the mixer in the setup page of UC
- By using the Interface Template for Quantum which has Cue Mix channels assigned in the output bus for you already.
Note: Bullet points 1 and 2 also applies to other DAWs (Logic, Pro Tools, etc).