CM,since you said that the distortion appeared from the main speakers rather than the center when you switched to stereo, it's pretty clear that the speakers aren't the problem(Curtis's comment was tongue-in-cheek, of course)since there would have to be multiple defects. A somewhat unlikely, but far less remote possibility is that both samples of the 525 are in fact distorting on center channel material. As Sat suggested, if you can possibly borrow a different receiver you could check that.

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Enjoy the music, not the equipment.