Zach, you continue to be tortured by what are almost certainly only imaginary concerns. Disregard what you read on that other forum from someone who may have developed a paranoid obsession about amplifier power. Stop kicking around either of those ideas, relax, and just enjoy your M80s and 635. No, an unused amp channel doesn't have any power which it can lend to another one which is in use. The power supply section of the amplifier supplies each channel with how much it needs.

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Enjoy the music, not the equipment.