Unfortunately its very, very easy to damage your hearing with music and headphones. It'a all about intensity.
Have a look at health&safety regs for a clue. 80-85dB(A) for 8 hours a day is thought to cause no more hearing loss than aging alone would produce. Up the intensity, and the time allowed drops dramatically; at 97dB(A), we're talking 3 minutes allowable on this criterion!
(reference)
The problem with headphones is that distortion is so low, and you have no external cues, that 'loud' doesn't sound like LOUD. But the damage is done all the same, and headphones allow 100dB(A) easily, whereas sustaining levels of 95dB(A) via speakers is PARTY LOUD. Scary fact: thanks to the walkmen of this world, the current generation of 15-20 year olds have hearing that, when they reach their parents age, will be aged an excess 30 years (ie 70+ yr old equivalent). I have references for this somewhere too...
The really bad news: no matter where excess levels happens in the audible spectrum, the damage happens first always starts with diminishing sensitivity in the range covering speech consonants (1khz-4khz); it's called presbycusis, and it's non-reversible. Eh? wot? didn't catch that bit.
The good news is that noise-induced temporary threshold shifts (when you can't hear sh*t for a couple of hours after leaving the club) after periods of excess volume are largely reversible; but this effect does not allow you to get away with going clubbing repeatedly. If your ears are ringing the next morning... there's probably some sensitivity lost. :-(
Take care of your hearing; it's not repairable. A good trick is to rub your finger and thumb together just next to the top of your pinna ( outer ear). If you can't hear it, the sound is probably too loud for prolonged exposure (>85db(A)).
Felix
(who uses headphones and horn-loaded loudspeakers...)