deniah gives good advice. The safe solution to being subjected to loud noise for several hours is finding some way to reduce to the noise, not blast through it with something louder.
Although I will say I once found myself on a very loud aircraft with nothing but a pair of insubstantial headphones that didn't even cover my entire ears. I ended up wearing a pair of foam earplugs and then putting the dinky headphones over them. Cranked the volume all the way up on my iPod to try and hear the music through the earplugs, didn't sound very good!