We loved our Amazon cruise, we did it on the Maasdam a smaller ship now sold. Lots of interesting things to see. Spend time on deck. Manaus is an interesting city, make sure you do an excursion to see what a rubber plantation worked.
It never hurts to ask all they can say is no. We were once offered OBC or a cabin upgrade, we took the cabin upgrade & went from an inside cabin to a balcony.