I won't go as far as to say adding something the Axe Fx can't add because I haven't explored that far with it. However, knowing the science behind a maximizer, I don't think the axe fx can do what a sonic maximizer can do. That's not to put down the amazing axe fx, it's just comparing the science between the two. The maximizer I use is the BBE 362 (the older version ) the newer ones has this grayish red theme to it.
But dude, the maximizer in this amp really sparkles. It's more noticed in the cleans for sure.