Source Audio Soundblox 2 User Manual page 10

SUSTAIN
There is a compressor before the distortion. The SUSTAIN knob adjusts
the input gain of the compressor. Increasing the SUSTAIN control will
result in a higher signal level going to the distortion, even when DRIVE
remains fixed. Increasing the amount of compression on the signal
effectively creates more sustain.
SUSTAIN
DRIVE
The DRIVE control adjusts the gain of the signal going into the distortion
section. You probably don't want to max out both SUSTAIN and DRIVE at
the same time, even for rock and roll.
DRIVE
10 | Controls: Knobs