Sully, it may be the sub amp, but another possibility could be the sub output on the Marantz. Is the sub trim control turned well up, say about mid-way? Have you previously had strong output using it with another sub? One way to test the sub itself is to temporarily connect a length of regular speaker wire(instead of the sub cable) to the high level terminals on the sub so that the sub out on the receiver isn't involved.

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Enjoy the music, not the equipment.