Click on the Ots Labs Power Amplifier in the OtsTurntables window. This will open the Output Configuration dialog box.
On the left-hand side of the Primary audio output section you will see the Cue chan: slider.
Click and drag the Mix slider down. This will remove the "ON-AIR" level in the cue channel.
Cue channel output options
How to set up your soundcard (main channel)
Output Configuration dialog box