What setting should my amp’s input sensitivity switch be set at?

I am hoping someone can help as I am not an engineering type. I have an integrated Vitus amp connected to a CD player via an rca cable. The amp has a sensitivity Vrms setting for each input. It has three settings <2, <4, and <8 vrms. Would anyone know what this should be set at? The lower the setting the louder it goes but I am sure there is more to it than that.

Thanks for any advice you can give.