NP, welcome. It's pretty much no-contest for the sub, but I'll add-on. Since you're talking about speakers(M80s)which require slightly less power than most and already have a quite powerful amplifier, it's highly unlikely that you're exceeding the maximum power capability of your present amplifier. If not, no amplifier, regardless of maximum power capacity or cost, can improve your sound. Get that sub and enjoy.

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Enjoy the music, not the equipment.