The DRM itself has no effect on the sound quality. The improved sound quality that the Plus tracks have are the cause of the better sound.
According to Apple, you can upgrade your existing music to better quality and DRM-free by paying them money… but I agree with many of the sentiments expressed above and say that I doubt you’ll be able to tell the difference in sound and that there are other free techniques by which you can remove the Fairplay DRM shackles.
Contrary to some reports, the AAC format is not Apple proprietary. It’s open and supported on a number of third-part digital music players. It’s capable of better sound quality and smaller file sizes. Good thing!
On the other hand, it appears science has found that “young” people actually prefer the poorer sound quality of MP3 what with the high-end distortion, pops, and pips.
On the other, other hand, I’ve heard the “older” people prefer the sound from vinyl LPs.
So, you can’t win.