Yes, I think you can see behaviors like the peeing on the nubblies...you need to break their habit and they want to continue with it! It will take months but hopefully you can eventually be plastic free on your sofa again! I would retreat with the Anti-Icky Poo... most behaviorists I have dealt with recommend enzyme cleaners for cats.
Clear litterboxes are usually tote boxes adapted for use with cats, when you said you had "giant" litterboxes so I assumed they were tote boxes, sorry! I've used what the store labels as "jumbo" litterboxes and they are still smaller than a tote box...but I was thinking more of high sides that might prevent a cat from seeing all around...and as that is not the case, this is probably not a factor!
Clear litterboxes are usually tote boxes adapted for use with cats, when you said you had "giant" litterboxes so I assumed they were tote boxes, sorry! I've used what the store labels as "jumbo" litterboxes and they are still smaller than a tote box...but I was thinking more of high sides that might prevent a cat from seeing all around...and as that is not the case, this is probably not a factor!