While it’s true that bass frequencies aren’t perceived until they hit the very inside of your cochlea, any frequency in a dangerous decibel range can do damage if it’s too high.
Also, since your treble hearing cells are right at the front of the cochlea, they take the hit regardless of what frequency is coming through. This is why treble tends to be the first range to go in Noise Induced Hearing Loss. You don’t want sound at any frequency level to be entering your ears at high volumes for too long.
DJ listeners tend to be more interested in the lower range of the spectrum than the treble, and we hear less bass frequencies at lower volumes thanks to quirks in our relative hearing perception.
So, DJ headphones tend to be tuned to be heavy on bass response. You’ll have an easier time hearing the bass without having to crank things up to super dangerous levels. Bass also tends to help with isolation in loud environments.
I’m not big into clubbing. Many club environments are stupidly dangerous in terms of volume levels, and a smart DJ will know not to destroy their audience’s ears, or their own.
If you try to crank up the average “neutrally tuned” pair of headphones until you can hear the bass frequencies well enough for DJ work in a loud environment (check out Fletcher-Munson equal loudness curves sometime) the treble will be so hot that it’ll be stabbing you in the ear hairs with knives.
The HD-25, perhaps the world’s most popular DJ headphone, has a very prominent and flat low and midbass response, a huge dip in the upper mids, and a treble peak that’s tuned to never jump out higher than the bass frequencies. It’s the perfect DJ frequency response. And they have solid passive isolation as well.
It’s more about being able to hear the bass without anything being at dangerous levels than bass vs. treble being more damaging. They’re all super damaging if they’re loud enough.