It all comes down to the pre-amp stage and the type of tube you're using for that, can't remember right off hand but I think it's the 12AX..something, and depending on the level of signal going to it (aka pre-amp volume control) you can get a clean sound or overload the tube to get the distorted sound. The you run that sound to your tone control circuitry, then the rest is all the power stage preferably a non-distorted tube sound on the output.