Do you know anything about the Innova brands or Solid Gold and another called Natural Balance?
When we all looked into it their duck and potato is a little high in copper and cockers are one of the breeds that "can" have a problem with too much copper in their diet.
Stay away from anything with BHA, BHT, Ethioxin. Also look for meat as first ingredient and preferably not meat meal, but real meat. Just about everything Onlynaturalpets sells is the highest quality and you can feel pretty safe feeding. Sitstay.com has good quality foods and treats, too.