I hate to tell you this but I'm agreeing with Villette, too
We had a similar problem to BLowry...the cats we had about 10 years ago started to go in the corner of our basement den...we tried EVERYTHING to clean that carpet.
Heavy duty commercial cleaners made specifically for cat urine did nothing to get rid of it...we tried a few of them and spent tons of $$ but it seemed that nothing worked.
We ended up getting rid of the carpet and underpad and had to scour the cement underneath which STILL maintained some of the urine smell!!
This was years ago.
Just recently, our newer cat decided she didn't want to go in her litter box anymore (daughter wasn't keeping it clean enough..it's her cat) so where does the cat go? In the same corner of the den in the basement!!!! The carpet is relatively new, so I know what the future holds for us....no more carpet in the den downstairs!! I'm going to put in laminate flooring as soon as I can afford it.
Good luck...hopefully there are newer things out on the market now and if you find something that's successful, I'd love to hear about it!