It isn't likely that it's the total cause of the problem, but yes, despite the direction in the manual that the 6ohm impedance setting should be used with 4ohm front speakers, don't do it. This reduces the maximum voltage that the power supply section of the receiver can output and, following Ohm's Law, this likewise reduces the maximum current and power capability. This can cut the maximum power capacity in half. This is a UL mandated safety feature that should never actually be used.

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Enjoy the music, not the equipment.