Were you, by any chance, eating crackers when you opened the box? Of course you should use a dehumidifier in your gun safe (especially with a Stack-On safe). Here's one of the better rods you could use:
Golden Rod, 12 inch, Dehumidifier Rod
Took me less than an hour to install one in the bottom of my safe; (Which is where it should be placed.) and, over the years, it's been well worth the money and effort. (I had to use a high speed drill in order to put a small hole in the side of my safe to pass the connection wire through.) After that everything is fully automatic; all ya got 'a do is make certain the plug remains in the wall outlet. You'll need to use a lot less gun oil, as well.
As for the stored ammo? Army ammo cans with gasketed lids are one of the best ways to store ammo for an extended period of time. (In cardboard is one of the worst!)