What do these vets mean by poor quality?  If you get the feeds tested for nutritional content at a lab, then you will likely end up with the same numbers that are on the bag in the "Guaranteed Analysis" section (protein, fat, etc).  Are they concerned about low quality ingredients?  You can read the ingredients list on the bag but testing won't tell you anything that isn't on the bag.  Perhaps they are concerned about contaminants in the feed?  You can send in a sample to test for mycotoxins or mold, but most feed companies have their own quality control testing.  

How will you determine which feed is the "best"?  There is no one best feed for all horses.  The feed you choose for each horse should be based on the individual animal's needs.  Heavily exercising horses need more protein than pasture pets.  Senior feeds are most appropriate for horses who are showing their age but not for younger horses.  Low carb feeds might be good for horses with insulin resistance or other metabolic problems, but are not necessary for every horse.  

Triple Crown and Poulin are both reputable feed companies that have their own equine nutritionists and quality testing, so I would not worry about their quality. With those companies, the most you should do is make sure you have selected the most appropriate feed choice for each of your horses. The local feed mill could go either way depending on their level of expertise with horse nutrition.  

If your horses are doing well on their current diets and look happy and healthy, nice coats, etc, then your feed is probably fine.