perhaps you might try swapping in your surrounds or another pair of speakers you might own in to isolate the problem? Rare as having issues with two speakers at once may seem, I had this happen once.
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Model 1070
Toshiba HD-A35 & Pioneer BDP-51
Tivo Series 3
Elite 50" / Aquos 32"
Squeezebox Radio

Stereo Setup:
NAD C326BEE (50Wx2)
Elite DV-47Ai Universal SACD/DVD-A
Citypulse DA7.2x II + TXCO DAC
B&W 685s
JL Fathom F112
Denon AH-D2000 Headphones