I did a couple tests today, one blind and one not, comparing a couple 196 kbps mp3’s against the same songs encoded in aac format at 128 kpbs. Even with the crappy headphones I have here at work, I could tell the difference. The acc encoded file sounded more rich and complete. The difference was most apparent at the bottom of the register. The bass in the mp3’s just sounded like something was missing.