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.
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I think you can do what you want with covers... record them etc. I do remember hearing that you cant sell them or something like that and you have to pay money to the people who originally made the song or something.. not too sure though.
