What is the best cat food for a cat diagnosed with urinary tract blockage?

My cats vet recommended Science Diet or Eukanuba with low ph. My cat had surgery several years ago for a blocked (crystals) urinary tract system. The vet at the time said he would always have to be on their special vet diet food (low ph Eukanuba or Science Diet). A new Vet has since taken her place and he has not given me a straight answer. I was wondering if anyone else knows of a food that is low in ph that can be fed to a cat that has had crystal blockage. Oh, before he had the crystal blockage, I was feeding him Friskies because that was all that he would eat. The vet told me that was what caused it; that and stress from getting a new dog addition to the family.