[samuelboy]

I just bought the M1HPA recently. My source is the Marantz CD6003 connected to the line-in connection. On its own, there's no problem. Then yesterday I loop back to my Marantz PM6003 via its line-out connection to the CD channel input so that I still can play through the speakers. I have both the PM6003 and M1HPA switched on, and the sound from the M1HPA is still normal. But once i switch off the PM6003, the sound from the M1HPA starts cracking up. It got so bad till it sounded like going to blow (like crushing plastic bag). I switch on the PM6003, the sound went back to normal. Switch it off, and it cracks up again, and so on.
 
Any idea what could be the problem?