[quote author="vertiges"]Does changing the values of the bias resistors (grid + capsules ones)
affect the output of the capsule ?[/quote]
Essentially no, but it affects what arrives at the input of the gain stage. And remember what Samuel said about how inevitably a filter is formed.
I think experimenting with guitar amp circuits gives you a good idea of how levels affect things when woking with valve gain stages. Good sounding amps pay attention to how much level you hit a stage with, and how much you expect at the output.