Had a look at the circuit last night and how the tube is used, and it certainly doesn't look like a gain stage to me.

Seems more like a limiter or compressor type thing. And if it is then there shouldn't be much difference in tone when switched in or out. As a limiter or mild compressor you would/should notice a variation or constraint in volume dynamics with the tube switched in, which I expect would be more noticeable at higher volume levels when driven hard.

Couldn't find a user manual or handbook so cannot confirm Carvins intentions in regards to the inclusion of the tube