Here's one for ya James. I work for the sound company that is touring with The Who, while I finish my flight training. Loud environments are definatly an occupational hazard.
Even with constant tinnitus, passing a class 1 audio exam wasn't a problem. That said, I'm rather cautious about what I expose my hearing to, and try to protect it as much as I can (which can be very difficult).
"is there any anecdotal evidence of one-off big noises giving a permanent step loss in hearing"? Yes. I once spent an hour wedged between a stage and crowd barrier with 10 other guys, trying to prevent it moving forward from a unruly crowd....with 3 trumpet players playing 1 meter from one of my ears....it's never been the same since.