You have several volume controls & two operating modes.
Make sure you are in DRIVE mode, not in CLEAN mode
Does the DRIVE led turn on ? If not you wont be able to get more "drive" and you will need a footswitch. You can make a footswitch with a stereo 1/4" plug & some switches. Short TIP to SLEEVE to switch between clean & overdrive, short RING to SLEEVE to turn off reverb
If you want more distortion in drive mode, with less volume, turn the DRIVE knob up, and turn the DRIVE VOLUME knob down.
DRIVE controls how much distortion / overdrive you get, and DRIVE VOLUME controls the overall volume.
CLEAN controls how mush clean volume you get.