Why did this happen when I did this (read more please)?

So I just found the equalizer in my iTunes and I started to play with it. I set preamp and 32Hz to the max and started to listen with my MDR-XB20EX (5 - 23 000 Hz) IEMs. I started to listen a song which I had previously bought yesterday from iTunes. The sound was distorded and crappy. But when I switched to listen an AAC (320 kbps) it was fine ((ish) slightly worse). Any idea why?