I don't know if there are any for the mac, but try searching for a LAME encoder. LAME encoders alter the bitrate according to the frequency of the song at a certain intervals. That way, the most rich part of your song will have a very high bitrate, while the parts of the song that are very regular or increase and decrease very linearly will be broken with a very low bitrate. Thus, your music has a better quality overall and you have a file that is still about the size of a regular mp3. The only bad side is that it takes a minute or two longer to encode using this encoder since it is much more meticulous. However, if you treasure quality over speed, I would suggest searching for such an encoder.