When studied in the response to reducing glomerulonephritis and protein loss in the urine, rubenal was shown to have better effects than the ace inhibitor. However when used together they had a synergistic effect and worked very well together. Here is a link for purchasing rubenal at HealthyPets.
I would recommend that the protein loss in the urine be reevaluated after amlodipine and rubenal therapy. If still present, then instituting Benazepril might be a great idea.