What does the “boost switch” boost? The schematic for the Two Stroke shows the cathode of V1 (the first side of the 12AX7 pre-amp tube) is biased with a 1.5K resistor to ground. The “boost switch” allows you to add a bypass capacitor to the 1.5K resistor going to ground. The bypass cap removes some of the feedback effect in the bias circuit and allows for more gain (and distortion). Adding the .47uF cap partially bypasses the the AC signal around the bias resistor, adding the 25uf cap passes even more of the AC signal around the resistor. So, you can see that with the “boost switch” off, the signal from the first gain stage is “un-bypassed” with the smoothest, lowest gain tone. Adding the .47uFcap increases the gain and produces a more edgy tone, selection the 25uf cap increases the gain further and provides a harder, more over-driven tone.

No bypass cap = blackface tone, .47uf cap = Tweed tone, 25uf cap = Plexi tone. There is more information about preamp circuits and cathode biasing available at the tone wizard website. There are a number of bias calculators available online that allow you to adjust the values of a preamp circuit (including the bias bypass cap value) and see how it changes the characteristics of the preamp gain stage.