I'd say your next best step is to Download EAC/Lame, and rip your demo again, making sure to keep the wav file. Then load the wav,

Beelzebub: This is the best advice so far.

If you want to do audio frequency response tests, don't use MP3s. Rip one test track to a WAV file, raw, without any MP3 encoding as part of the process (i.e., don't decode an MP3 and think you've made a good WAV).

Since the empeg can play pure WAV files, it can play files which are bit-for-bit identical to the audio that your CD player is reading. So use WAV files for testing.