Yes you can add an amp to your setup and it most likely will improve on a few things provided that the amp you add is of better quality than the Marantz's onboard amps. It will take some of the load off of the Marantz power supply which will allow more power to go to the remaining Marantz amp channels.
How much improvement is tough to determine without more specifics.
Bill
How much improvement is tough to determine without more specifics.
Bill