Crystals in cat urine

This seems to be the month of vet visits. Hannibal, our 9-year old black cat had to go into the vet for crystals in his urine that plugged his urethra. We have had this problem before, but we thought the food he was eating was low ash/mineral. Male cats who 1) live in-doors, 2) are neutered, and 3) eat too high a mineral/ash diet are very prone to this condition. I guess that their urine is not acidic enough? We just changed his cat food to a special urinary diet.

We just love our cats so much, and can't let them out--we live on a busy street.(My friend's cat was killed a couple of weeks ago.) But the down side of keeping them indoors is that they don't exercise as much. Our third cat, Vanguard (he's the model for my avatar) has managed to solve the problem himself by spraying on all of my potted floor plants. I can't even get too upset--most of it gets in the saucer and he's never had the crystal problem. He's also 16.5 years old, and still going strong.

Has anyone had this problem with male neutered cats, and if so, what suggestions do you have?