The Marantz A/V recievers typically need 2 setup codes in UEIC (URC and some RS) remotes in order to achieve all functions. With my SR-5300, I need to use the AMP 0189 and 0269 codes. The number buttons usually hold the device selections, and are a better indicator than Power of whether the code is correct. Try these two codes and, with the amp turned on, try the number buttons to see if they select an input. If either of these codes work for even a couple of commands, that will be a start.
Jim