Bryan, the surround crossover measurement is a notch higher than typical, but apparently in your mounting location that's their bass capability. Although the M60 measurement, as you say, apparently showed good enough response at 40Hz for the "large" setting, that doesn't mean that it's the best overall result. As Dr. Kyriakakis suggests, the mains should manually be set "small", with perhaps an 80Hz crossover, so that the sub can handle more of what it does best.

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Enjoy the music, not the equipment.