I searched thought all the forums and the answer to my question is to do the following :
DepthOfField dof;
GameManager.Instance.PostProcess.sharedProfile.TryGet<DepthOfField>(out dof);
if (dof != null)
dof.active = true;
(GameManager.Instance.PostProcess is the Volume component, linked from the scene)
But this does nothing inside my project.
What did I miss ? Is this a bug from Unity 6 ? (my project is in 6000.0.15, also tested in 6000.0.34)