Samsonian...
I'm kinda late comin' to this thread, but I would second DCing all the supplements while you're changing foods, esp the fish oil. I have 3 and all of them get sick on fish oil. Diarrhea and vomiting are the usual results of my adding this to their food. It takes them a while to settle down, too. And my Boxer can't eat red meat, esp venison. Gets sick every time.
Some of the foods you mentioned trying are pretty high quality for commercial foods, so you're not trying junk food. Caifornia Natural Lamb & Rice or Chicken and Rice has been good for one family I know.
I remember someone else on the boards here using Wellness Core. It's grain-free but the protein level is around 34% so they used it for their pup.
Good luck with Samson...