As an example, on the Fender Twin model, cranking the Drive doesn't seem to do much. Now that said, a Twin is a no-master-volume amp, so maybe this is an intentional part of the model's design. The rather spare DT25 documentation doesn't say much, other than Drive controlling "dirt," but there isn't much dirt to be had on some of the amp models. Is it just another volume on those? Or does it only work at high volumes?
That is correct a Twin just gets louder and not much dirtier. No real gain stacking going on.