Yeah, I did the same thing it works. Just be super careful. They don't want to give up the tension (which is probably a good thing), so you need to go slow over time really.
I'm considering getting the 380, as I'm guessing they are an improvement, but the isolation with the 280 (even stretched a bit) is very good.
Of course the reason they clamp so much is for isolation. New, these are extremely isolating.
Funny though, i've had these for quite a while and the headbands do NOT hold up. The plastic breaks, and screws fall out. No big deal. Nothing good ol duct tape can't handle. I'll bet there are lots of these sporting duct tape across the globe.
Most of them probably do. =p
HD380 is more durable and sounds better, but does not isolate as well.
It's probably best to not bend HD280's headband for improved comfort, that really only works with headbands that have steel backbone.