Just wanted to say--consider that the rather similar V4b has no sensitivity switch. For maximum headroom, it's got no cathode bypass cap when stock, which is equivalent to being hard wired in the -9db position.
This allows for 'higher output' instrument level signals--like some active basses provide on transients, or a true "clean boost" pedal might yield--to remain clean at points when the other settings might begin to clip to some degree.
That said, if you're not clipping that first tube stage, the setting probably matters very little unless you are trying to drive the amp into clipping, and/or have the volume set pretty high.
Once you enter crunchy territory with the amp, the interaction between the sensitivity setting and the volume knob can theoretically provide some subtle differences in tonality, as the volume knob also functions to filter some highs a bit when not maxed, as it provides some series resistance to the grid of the next tube which shaves some high end, and if the bright + is engaged, volume knob setting interacts with that tonally, too.
One other thing is, lower sensitivity settings might make the volume pot seem less abrupt in its taper at lower settings.
This allows for 'higher output' instrument level signals--like some active basses provide on transients, or a true "clean boost" pedal might yield--to remain clean at points when the other settings might begin to clip to some degree.
That said, if you're not clipping that first tube stage, the setting probably matters very little unless you are trying to drive the amp into clipping, and/or have the volume set pretty high.
Once you enter crunchy territory with the amp, the interaction between the sensitivity setting and the volume knob can theoretically provide some subtle differences in tonality, as the volume knob also functions to filter some highs a bit when not maxed, as it provides some series resistance to the grid of the next tube which shaves some high end, and if the bright + is engaged, volume knob setting interacts with that tonally, too.
One other thing is, lower sensitivity settings might make the volume pot seem less abrupt in its taper at lower settings.