If the C/D alone isn't getting the Ph level down far enough, ask your vet if there's a possibility of also giving a tablet called Methioform in addition to the food.

Methioform is specifically for Ph balance and two of my cats were on it to lower their Ph before we switched them over to the C/D food. Both my cats (Bear & Cami) really liked the stuff too, so it wasn't a pain to get them to take it.