To get the heavy distortion press the CHAN. button and set the PRE knob of the LEAD all the way to 9 or 10. Now turn the POST knob up to 2 or 3 and see if you like it. You can control the extent of distortion by setting the PRE to high or low within the range of 7-10.
I hope this helps. Someday we'll all own the Marshall stacks...;)