I definitely believe there are different qualities of food.
I was feeding Eukanuba and thought it was great until I learned more.
Then I switched to Canidae and it was far better. Until...they got bought out and switched ingredients and stuff recently. This was an ordeal, and took me a while to figure out that it was the Canidae food and not something wrong with my dog. Major diarhea etc for a 100lb Ridgeback is no fun. This went on for a couple of weeks until I googled canidae and found out about theh switch.
Now I am back on Eukanuba (which I can tell is inferior but stable) but going to switch to Innova slowly for the longterm.