This is the first time I've ever done the comparison. I put on "Wishful Thinking" by Wilco, a song that is fairly simple, but has a lot of subtle sounds kinda floating in the distance. The difference between the two is far from minor. After just a second, it was like I was listening to two different recordings. The song coming from the CD was crisp and clear, while the crappy AAC version was muddy and flat.
I either need to re-rip my entire catalog or just punt my computer.