Isolate the problem. The amp sounds OK WITH the reverb pan? It sounds bad WITHOUT the reverb pan? Unplug only ONE of teh pan cords at a time. Pull only the INPUT (drive side) cord. Does that affect the sound? Return that cord and now pull the OUTPUT (return side) cord only. Does that affect the sound?
Just guessing here, but my first thought is that when you unplug the reverb pan, you are leaving the high gain return circuit unterminated and it is free to oscillate. The simple cure for that would be to connect a short across that return connection.