Okay, so my "first thought" about playing them too loud isn't it, and there's "gross distortion" even when they're reduced to a lower level. Another thought is to switch your main speakers(no problem now?)to the surround channels to see if the distortion now appears in them. If so, although it seems highly unlikely, it would appear that the surround(and center)channels are defective while the main channels are okay. The 5007 should be replaced. You can dismiss the thought that the new 5007 amplifier is "more revealing" of equipment problems.
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Enjoy the music, not the equipment.