In the Filter bar, set New Topics to Off. That should display latest replies/comments in the activity stream. I remember there used to be a module setting for this in Sitebuilder to set the New Topics filter to Off but it seems to be gone now.
Well it does show new replies rather than JUST topics. The issue I am having, is it isn't moving the reply to the top of the list. So if some1 replies back to a topic a few days old, they will never see it in latest activity. AND it doesn't bump old topics inside the channel back to the top either.