I don't think the diaphragm can go bad that easily. I have opened up many headphones with the imbalanced issue (including my own DIY sets), and I have never seen a burnt diaphragm just yet. For some reasons, even a small arc spot on the diaphragm can cause the driver to sound lighter. The quick and easy fix is to recoat it.
About the diaphragm material, I don't know what Stax uses. But, I am quite sure that your diaphragm is still good so there is no need to worry about replacing it. What you might need to replace could be the dust covers. They are extremely fragile.
Wachara C.