DK, theory would call for a thicker wire for runs of that length. Wire having slightly too high a resistance doesn't bother the amps and any lowering of sound level can be compensated in calibration. The interaction with the swings in impedance of the speakers would cause slight matching swings in frequency response in the speakers, but this might not be noticeable with the material reproduced by the surrounds. My view would be to not go through the bother of making the change.

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Enjoy the music, not the equipment.