Read this link struvite vs oxylate and diet change. Then discuss it with your vet or get a second opinion on going nuts trying different foods. I have gotten rid of high PH and struvites with crancaps (dbl dose for 2 weeks) and never changed food.
Bladder Stones + Crystals | B-Naturals.Com Newsletter
I know we spoke but please remind me if your pup has an RV (I get so many calls and emails its hard to remember). This is a first hearing struvites and hi PH can be genetic. I think your vet is grasping because they don't know what to do.... therefore suggesting a food change. And the fact that they don't even have a definitive answer as to what food to use tells me they are grasping.