Well, yeah. Here's what I did.
I connected the green and black plug from the speaker to the rack that comes with GTXP. I think the green one goes to the main and black one goes to the rear speakers. But then I tried playing mp3 but sound is coming from the front, left speaker.
I tried playing CDs with my Rio Volt, and the speaker played all 4 speakers perfectly. So the problem isnt with my Logitech Z-560, huh?