Substandard mp3 encoders are known for having a high freqeuncy rolloff which could be causing some of what you're hearing. I'd say your next best step is to Download EAC/Lame, and rip your demo again, making sure to keep the wav file. The load the wav, the lame mp3, and your orginal mp3 and compare them and see what you think. If the wav file sounds "right", then it's the mp3 compression. If the way file has the same differences, then it's still probably something you can solve with the equalizer. In the long run EAC might not be one click, but it really isn't labor intensive and will probably save you from ripping all your CDs again.

Matthew