I can't give any better advice than what is in dogaware. This following is from that article of Stuvite stones...
"Struvite crystals do not require a change in diet. Because struvite crystals do not pose a problem unless the dog has a urinary tract infection, there is no required treatment for crystals, including dietary changes. If the dog does have a urinary tract infection, a prescription dog food will not cure it.
If your veterinarian finds struvite crystals in the urine and suggests a diet change , you'd be well advised to find a new vet. You have to wonder how many other things he or she is misinformed about. It isn't just a case of not keeping up with newer research; this recommendation is just plain wrong."
I know how hard it is to go against your vets recommendations, but sometimes you just have to, to do what's best for your bully.
Been there and still am.
I pm you the link straight to the right page.