Get the smell out

[Goalpost]

So my question concerns washing. I have two cats. No, I am not going to get rid of them because of this. Their litter boxes are in my spare bedroom. There is a 'throw'/small blanket that I fold up and place under the boxes. Every month or so, I replace the throw with a clean one and wash the throw. It can't be helped that they smell like urine when I do this. I guess they come out of their box from digging and burying etc and walk on these throws 1st hence their is a smell after time.
So anyway I wash them. I have tried both...a regular wash and a pre soak cycle. I have used cheap detergent and better detergent. I dry them and fold them up and place them on a shelf in the closet, and guess what, they still have a urine smell. It's frustrating to wash and still have that smell. I have heard baking soda might work. I have heard fabric softener added might work. I know there are things on line that I can order but I really don't want to buy and deliver a cleaning product this way, it's a nuisance...although I might have to. About a month ago, one of my cats had an accident on the bed. I think it was a one off, because it hasn't happened since. Anyway I washed that comforter, placed it back on the bed, and guess what, it has a urine smell still also. Anybody have any secrets to wash and get the smell out better than what I am doing?