If you do a search "EVO" here at JL, you will find a lot of information that you might not get if you just take what people post in this thread.
As for your other question, re: food and ears, many labs are sensitive to a variety of foods, including (but not limited to) wheat, corn, and soy, which are present in many dog foods. These allergies often manifest themselves as persistent ear infections.
Ear problems can be caused by all sorts of things and not necessarily by food. There is often talk of food allergies but food allergies themselves are actually pretty uncommon. Intolerance's are often a more likely cause of food related problems. As already mentioned, some dogs have issues with certain ingredients -- sometimes that can be grains (wheat, barley, etc.) or meat sources (beef, pork, etc.).
The Common theory is that foods high in starches promote the growth of yeast in the dogs ears. There is also the possibility of allergies. Some people make the assumption that dogs and people have the same dietary needs. Dogs and wolves don't eat much if any carbs in the wild, but they can survive on them. fruits and beries are the simple sugar plants that they can gain the most from and things like grains (rice corn wheat..etc..) are the things that they have the hardest time getting nutrition from. The foods like Evo contain no grains and are really rich and usually you have to feed a much smaller amount than foods with grains in them. Grains are also called fillers in the dog food industry.