Let me make it clear, having AKC papers DOES NOT mean a dog is a quality animal. It DOES NOT mean the dog is worthy of being bred. ABC, or the American Boxer Club, is the official consultant of sorts to the AKC about what it takes to be a quality Boxer. ABC is the organization that sends the blueprint to the AKC, by which judges at dog shows rate quality. ABC is the group that does not recognize white Boxers.
Shows are where quality is measured.
If the champions are no closer than 4 or 5 generations away, it is very possible that the dogs in the subsequent generations were not carefully bred by a knowledgeable person with a well thought out plan for improving and maintaining the characteristics that make a Boxer a Boxer. Obedience degrees, and other measures of temperament also provide information, and if the breeder can also provide information about family health history, screening for hereditary diseases, etc. So, again, when looking for a puppy, look for champions very close in the pedigree, hopefully father or mother or both. Avoid breeders who try to sell you puppies from parents with disqualifying faults. In Boxers, disqualifying faults would include fathers who only have one descended testicle, and especially any puppies with one or both parents who are white.