I have witnessed options #1 and #2 in action successfully de-denting a cone. Though #1 didn't involve a cardboard tube; just lips directly on the dented cone.
I've heard of #3 & #5, though have never heard of anyone needing to go to that much trouble as 1 & 2 usually work.
There's no way I'd do #4.
If it really bothers you (it would drive me insane) and you can't pop it, option #9 would be to just call Axiom and buy another driver and replace the dented one. I'm pretty sure they will sell you an individual driver, and I don't even think they're that expensive. Yes, yes, overkill for a problem that doesn't affect the SQ, but if you're really picky about how your gear looks, it is a solution.
Good luck.
