As the Marantz isn't a true seperate amp, I would plug the RCA's from the Yamaha into the AUX input on the Marantz and then hook up the M80s to the speaker outputs on the Marantz. You will obviously need to set the Marantz to Aux input and then from this point you will have to level match all the speakers for the surround sound. I would try the Marantz at 1/4 volume first and then adjust its volume up from there till it reaches the same level as any of the other speakers which are hooked up to the Yamaha, from this point you should be able to use the Yamaha to control the volume.
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Jason
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My HT