As far as i know there is no way to change this, as the material viewer in the hypershade is not connected to the render settings it's simply a preview.
I never use it though i prefer to use the viewport arnold view as this is connected to the render settings.
Hopefully someone that uses it might be able to help more
Dave
From a readers' Q and A column in TV GUIDE: "If we get involved in a nuclear war, would the electromagnetic pulses from exploding bombs damage my videotapes?"