You already have a pre built in to the PM15S1. Why would you need another? Surely there are enough inputs for those sources on the PM15S1.
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As Rodman said, your Marantz integrated amp contains a preamp (source selection/level control). Besides, to do what you're suggesting the Marantz would have to provide main amp in jacks, which it may.
Your approach has merit in that it allows you to incrementally acquire a possibly better preamp and power amp than those contained within your integrated amp.