So now I'm re-ripping all my CDs at 320 kbps CBR, as I found that the Amazon mp3s I've purchased (at 256 kbps) sound fine.
This makes me love my car even more, as I can't tell these mp3s sound so bad on my headphones or computer speakers. Although generally headphones generally means I'm on the shuttle or an airplane. I should get new computer speakers.
Not sure what I'm going to do about all my low-bitrate mp3s that I don't have the CDs for. Used CD stores today, probably.