Rob, there would appear to be a question as to whether there would be an "upgrading" in reality. If your 990BX is still in good condition you should be getting excellent results with your M80s. The 990BX is rated at 330 watts per channel into 4 ohms. Extremely low THDs are fine as an exercise in technological excellence, but what you already have is far below audibility THD-wise.

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Enjoy the music, not the equipment.