Welcome aboard~ WOW, 2 puppies! How old are they? Can you post pics of the pups? Are they your first TMs?
Generally, mastiffs are allergic to grain and chicken found in kibble.
You can check www.dogfoodadvisor.com
for dog food ratings and customer feedback. Mastiff puppies should eat Large Breed adult food or All Stage food because puppy food has too much calcium causes fast growth. Slow and steady growth for a healthy mastiff. Protein in the food is not an issue unless the pup has kidney issues.
Dog Food FAQs: Protein
Check out Acana Regionals, Wellness Core, Castor & Pollux Grain Free, Earthborn Holistics, Fromm*s, Innova Natures Table, Victor*s Grain Free or Taste of the Wild grain free. Chewy.com is a great place to order dog food and they offer free shipping on orders over $49.
You may want to check out the raw diet for your pup. Raw diet can be fed to young pups. Check out the sub forum on raw diet that has a wealth of information. It is not recommended to feed the pup kibble (one with grains) and raw food. Do your research re: raw diet and form your own opinion.