Amplifiers contain voltages high enough to Kill you even after the amp has been powered Off !!! If you don't have enough electronic experience to identify and discharge the Filter Capacitors which contain these Deadly voltages, take your amp to a Qualified tech. If you don't know a qualified tech in your area, post a request in the alt.guitar.amps newsgroup, the folks there will be happy to help you.
1. First Option: From the bass control to ground you'll find a 6.8k resistor. Disconnect this resistor and insert a 25k pot. This effectively gives you an adjustable midrange control.
2. Second Option: Install the 25k pot as above, adjust it to where you like the amp's tone, measure the resistance, and then remove the pot & install a resistor of that value. This permanently customizes the midrange to your personal tastes.
3. Last Option: Just remove the 6.8k reistor altogether. This will leave one channel with the stock bassman sound. And the second channel will be permanently overdriven.