I talked to the main headphone design guy at Sennheiser about this at the CES show. I was interested in putting in a very low frequency boost switch on headphone amps. You'll notice that almost all cans start to roll off at about 100 Hz. I thought it would be interesting to have a LF boost that starts at 100Hz and then is about 6dB up at 20Hz. I asked him if he thought that would be troublesome in terms of damage for most cans. He said they test them at unbelievable levels (don't remember what the number was but it basically drove the coil out of the magnet in one direction and bottomed out in the other) for long periods of time (days) and he said it didn't hurt the cans at all. Now, that's Sennheiser cans, I don't know about others, but it seems to me that you're probably pretty safe in terms of damaging headphones.

Damaging your ears is another thing; you don't want to go there.