The CF6 is 'cone elliptical' in shape and that is the method used to deter ice formation. (my notes say by 'aerodynamic design')
The RB211 as you point out has the rubber tit! which by design would 'wobble', and shed any ice should it attempt to form as it gets out of balance.
JFI if that rubber tit is missing it's a 'No dispatch'.
The last of the original big fan engines with spinner core A/I was the RB211b2 and c2 that had hot air fed through the LP shaft that exited through the spinner cap.
