If the clip light is not turn on, and you are getting harsh nasty distortion, turn the Volume control down. The Volume is really the Input Gain control, and sets the level through preamp and tone controls. My experience is the front end of these amps can really take a lot of signal before they clip.
After the tone network the amp has a Boost Control just before the Boost Circuit. The Boost Circuit is a single FET designed to emulate a tube being pushed. If you want compression and/or a bit of OD, this is the part of the signal path you want to develop it in IMHO.
The Boost Control is simply a variable attenuator. In other words it reduces whatever signal is coming from the preamp. This means there will be interaction between the Volume Control and the Boost Control, I.E. each effects the signal level going to the to Boost Circuit.
Here is the schematic that shows the Boost Control and the Boost Circuit which is labeled Q1 J113
Short Story: The signal come out A and goes through a few components then goes to C. The signal is taken of the top terminal of the FET and routed to the Woofer Master. Depending upon the signal level the FET is either clean and punchy, compressed, or grindy.
A bit longer story: The signal come out of the Boost Control at A. The signal goes through an Op Amp and is split to go to the Tweeter HPF and Depending upon the position of the Woofer Croossover switch, it is either routed back directly to C or sent to the woofer LPF, and then to C.
