About the clamping, just manually stretch out the headband slightly, a few times, on a few occasions - that did the trick for me. Plus, I just put a small diary where the headband meets the clamps: this also stretches them out and I think 2-3 days of doing it fixed things for me. Please note that while doing this, they're stretched out about the same as they woudl be on my head, so not sure why using them for 3 months hasn't fixed this for you. But do try the headband stretch: it's metal so you can't really harm it.
I gently manually stretched my headband every few hours over the course of a couple of evenings. It took all of the clamp out of them. Another trick I read was to take a pillow and fold it in half. Then put the DT880s over the folded pillow for a couple of nights. Some people suggest using a stack of books, and I tried that initially, but soon thought that it would flatten out the pads. I ended up just stretching them by hand.
Edited by Spiral Out - 4/5/14 at 11:52am