When you play a low-quality MP3 (96kbps or 128kbps) on a large PA system, the compression algorithm strips away frequencies. You lose the punch of the kick drum and the sparkle of the hi-hats. When you slow down or speed up a track (pitch shifting), artifacts become even more pronounced.
Many "free" sites take a low-quality YouTube rip and "upscale" it to 320kbps. It looks like a high-quality file in your software, but it sounds terrible on a big system. beatport download toper 320kbps