If you check out the Thread at
http://mtu.com/vbulletin/showthread.php?threadid=55 you will see that MP3 files will damage the audio voltages in the left and right channels independently, and thus create an incomplete cancelation of any center panned (equally loud voltage present in the left and right channels at the identical point). Your experimentation proved this.
Thanks for this post. I hope others will learn from your experience.