How is the 5150 II you've got ? I'm thinking of getting one !! Could you put some sound clips up somewhere so i could hear what it sounds like? Surely with that much distortion you should be getting good pinch harmonics anyway!
By virtue of their electrical properties, tubes generate a special waveform when they're saturated, which is why tube engineering has tremendous tonal advantages over solid state or DSP solutions, particularly for crunch and lead sounds. Tubes enter the saturation zone gradually or softly, which lends tube-driven tone its trademark yet totally unique character.