bass face has two definitions;
1) a musical variation of the poker face
in which the bass player of a given band wears an expression lacking any noticeable emotion, giving them an anonymous, emotionally distant appearance.
2) A bass face is the name given to the face a DJ or audiophile
when a beat drops or a bassline is particularly compelling and gritty. This use of the term is used for electronic music, especially drum & bass, dubstep, and any other dance music where the main interest is the bassline.