Lisa, we all would love for your birds to recover. A vet will treat your birds because you ask him to, but what he may not do is tell you exactly what they have (many guess rather than test anyway) and he will most likely not tell you that if they recover, they may remain carriers for life. Obviously, to know it's one of those diseases, they'd have to test the birds. Many, maybe even most, respiratory chicken diseases are similar to herpes/HIV type viruses in that regard-the body retains them for the rest of their lives.
That is why many have the policy to cull their birds who come down with respiratory illness rather than treat. It's not that they don't care about the birds; they just do not want to set up a cycle of disease in their flocks that they may never be free of. Anyone who knows me or has seen me around my birds, knows how very much I love them. Even then, I would not treat, per the policy I put into place before I even got my first chicks. So, I don't want you to think that I don't care about my birds and am just being flippant and insensitive when I mention euthanizing sick birds. Naturally, it's your choice and I hope things turn out okay for you.
I think you're wanting another antibiotic to try and I do understand your desperation. The one recommended for respiratory stuff is Tylan injectible. Since they still have those symptoms after a round of Duramycin, I feel it's probably viral, though, and you may be building antibiotic resistance by treating with an AB when it isn't necessary. I am certainly not a vet and do not claim to have poultry vet's knowledge, but I would dare say that most experienced poultry keepers here on BYC know more about chickens than the average vet does. The "treat or cull" is your personal decision and no one is trying to take that away from you. Some diseases are passed down through the egg from carrier birds, though, and you should just be aware of that.