@MarkPAman
I just looked at it last night. Maybe I misunderstood the button on the usb symbol and what the knob under it was for.
It certainly looked like preamp gain same as you do in a screen with the other knob when you select the dB level subscreen.
@MikeC
No idea which usb it connects too, but would have guessed that it would be the usb you had selected as shown in the block diagram.
But still wonder where you see a meter or screen that tells you the level it is at.