In case of a calibration error "Signal to low":

1. Check the cable connection between x1S<->audio interface
2. Increase the audio interface headphone volume.
3. Increase the audio interface input gain for both inputs 
4. Decrease the sample rate
5. Use ASIO driver instead WDM  (stute.x1A Setup audio interface) or vice versa.
6. Disable all sound enhancements of the audio interface

Connection stute.x1S - audio interface 

Connect the x1S "Input" with the "Headphones" output of the audio interface (Cinch cable, Red=Right channel). Connect the x1S "Output" with the audio interface "Input 01/L" and "02/R"