PRESONUS BLUETUBE User Manual page 7

Tube for signal input from mic XLR or instrument ¼
inch input.
Drive.
The Blue Tube provides a Drive
potentiometer on each channel for controlling the
amount of signal routed to the 12AX7 vacuum tube.
This feature lets you control how much saturation of
the signal occurs. Greater levels of tube saturation
give the signal greater warmth and a richer sound.
This works equally well on mics and instruments.