I'm guessing you are connecting to the Marantz and staying connected. For some reason if you are connected to the Marantz when Crestron reboots, Crestron can not reestablish the connection until the state of the Marantz changes. I'm guessing at this point is when the Marantz realizes the connection is no longer active and closes it.

What I have done in the past is, just after program start I send the http commands to power it on and then back off.

It also helps a lot if you only have the Marantz connection connected when it is in use.