This is such a familiar problem for me. I have been doing much the same with scrubbing, scrubbing, and scrubbing the boxes I have to get them as clean as possible and putting in all new litter.

I had tried the Dr. Elsley's Senior Cat Attract litter, but it was so darn dusty, I couldn't stand it.

What I'm using now is MIMI Crystals, which is a 4lb bag, and is about $4 from Walmart. I have those ginormous pans from PetSmart, and I put one bag in each box. I changed the entire contents of litter every 3-4 weeks. I've been finding as long as I keep the pans super clean, especially the one I have in the kitchen for Boris (my main pee-er outside the box), it's been much better. I thought I should have been buying stock in paper towels, as there had been pee on the stove and countertops at least 3-4 times a day. I was just so frustrated.

SO, new box in kitchen. It gets totally washed down every 2-3 days, and since it is a sifter box, I can change the bottoms and let the one air out really good as well.

Hope things will continue to improve on your front.