There's a few videos or clips around I remember watching a while back, but don't remember exactly what they are. Foam seems to cut down on some of the lower frequency resonances. It makes the cab sound more clinical and controlled but at the expense of some warmth, if I remember right.
Depending on the guitar tone, less or no foam could help give some extra body to a thinner tone, while more foam could de-mud a slightly more bloated tone.
Somebody please correct me if I'm wrong, again it's been awhile since I've looked into it.