Each valve contains 2 triodes which can be used as gain stages but they can indeed do things other than being stages that add to distortion, although they may still contribute to the overall character of the circuit.
Some valves will be shared by the clean & overdrive channels in some amps. For example, in the SLO V1 contains the main shared input gain stage for both channels and the second stage for the clean channel, while both halves of V2 are only in the overdrive channel. V3, V4 and V5 are shared by both channels.
That explains why if you ever look at an amplifier schematic, you might see V1A and V1B labeling two halves of the same valve.
In the Avenger, half of one of the preamp valves isn't used for anything. A loopless SLO doesn't include V4, which is a recovery stage for the loop and a cathode follower to drive the tone stack. In the Avenger and loopless SLO, V3 is used as a gain stage, and tone stack driver.
Now to further complicate things, in the SLO, the channel switching method allows bleed through from the clean channel to the overdrive channel, so technically speaker the half of V1 in the clean channel can be heard in the Overdrive channel. It's more apparent if you use the clean channel in crunch mode and crank the gain.
There a LOTS of arguments to be had over what makes that circuit sound awesome and I'm glossing over a lot of it, but essentially, one 12AX7 can be two gain stages and can be shared between channels in a multichannel amp. sometimes manufactures make it clear (VHT springs to mind), and sometimes they don't (Peavey 5150's spring to mind)
