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. Replace the stock Ceramic Coupling Cap at the Phase Inverter's Input with a 0.01 Polypropylene, Polystyrene, or Mylar cap. Let your ears be the judge as to which composition you like best.