1) Clean the box to the walls once a week, and spray with Nature's Miracle, letting it dry before refilling.
2) Check to make sure you have no 'stains' around the cat box, and no littler spilled around it. Doesn't take much to become pungent.
3) Consider changing your litter to Fresh Results. It's pine based, so it doesn't hold odor as much as clay litter. It's also clumping, and much lighter, which makes disposal easier. And it's biodegradable!
4) Rather than pay extra for deodorizer, try sprinkling the bottom with baking soda, it works better, is less noxious to the cats, and cheaper, too.
5) Try a covered box with a built-in filter slot, this does lots to cut down on the smell.
6) Get another litter box! For some reason nobody understands, it's best to have one more than the number of cats you are hosting, And it cuts down on the concentration.