Actually, I have used RCAs for balanced circuits - the only thing is that they have to be isolated electrically from the component's chassis. A balanced line comprises two conductors: a ground reference is not necessary. Therefore a two conductor jack (if you count the shell, which is normally grounded in an unbalanced circuit) such as an RCA or BNC can be used, again, as long as it's shell is not in contact with a grounded chassis.