Greg,
Lets do one thing at the time, Multi pass scan without any other filter function like grain reduction. If there is any electronic noise responsible for the image quality, the signal to noise ratio should improve by a factor of two, if we do four passes.
I can not see this with my FS 4000. It seems to me, if there is not a noise problem with a particular scanner, the limitation is still the grain of the film material. I did the test with very fine grain negative film. An other source of extensive grain is under exposure. Today's modern negative film's need one stop over exposure in respect to the recommended ASA value. This means, a 400 ASA film is in reality 200 ASA. Under exposes areas tend to be very grainy.