Maybe the box itself needs to be cleaned.

We had a warm sunny day here and I spent today scrubbing litter boxes and letting them air out in the sun. It helped the overall smell tremendously. When it was just Ripley or even Jazz, Scout and Ripley the boxes didn't get very dirty themselves but now that we've added the kittens it seems the boxes get icky fast. Right now I have 3 different kinds of litter in 6 boxes trying to see which works best for us. I have Tidy Cat, Fresh Step and Esquisicat from PetsMart. So far Tidy Cat is the worst for building up a gunky layer on the bottom of the box and it doesn't clup well, Esquisicat is entirely too dusty but clumps well and has no perfume scent and Fresh Step is kind of dusty, has a nice scent but it's too strong for me and I think a few of the cats because it gets used the least. I don't know which I will pick for my full time litter.