On the AST1 there is a control called 'Sensitivity'. This is a knob that compensates for the different outputs of various pickups. If you turn the volume up to full, you can have the amp break up like a Plexi by leaving the Sensitivity set high, or retain a clean sound by setting it lower to counteract the extra drive from hotter pickups. It is not simply an extra volume control.
Only the AST1 has this facility. The other amps clean tones will break up like a Plexi when turned up full with hot pickups.