Music for DJ use didn't used to be very critical in terms of noise floor as the surroundings were anticipated to be pretty noisy, unlike a quiet home environment.
Very true. When I started, PA amplifiers were not very powerful and distortion was the result.
Most DJ's play in loud background venues and the young audiance do'nt seem to mind.
I played to an older audiance; yacht clubs, country clubs, coporate events etc. that tend to not enjoy loud distorted music. Most DJ's buy used or inexpensive equipment that is usually underpowered for the venue. They crank up the amp to get the volume and bass thump which introduces distortion. Distortion is what causes the party goers to get "headaches" or perceive the sound to be not good.
I buy high end DJ equipment, as stated previously. I have 1200 watts of amplification to my two main speakers and my two subwoofers are self powered. When I play this in a medium size venue, the amp is loafing and minimal distortion is experienced. My goal of playing with minimal distortion is not only for my clients enjoyment but also has a self preserving purpose. Listening to loud distorted music for a 4 hour gig would make my ears hurt or ring. I've developed tinnitus over the years and poorly reproduced music makes my years ring and hurt. Because of the additional headroom, there is more punch to the music. I often play jazz and big band which is more demanding than rock and pop.
When I play a large venue, I wear hearing protectors .