I am of the opinion that an integrated can sound exactly the same as a preamp/amp combo if its designed correctly. Possibly better since the cable runs in the integrated are shorter than wires on the outside.
Having a separate pre is good if you want to use different amps one day, I think of it as a matter of convenience. If your amp goes down, you can substitute a different one, same story goes for the pre. You'l likely have a higher resale value for separates if you change out your equipment often. Plus, there are some special pre amps that have functions most receivers don't have.
As for the marantzes, you might have to study the topology of both combinations and take measurements. The amp boards may be identical in some cases, but simply split up and housed in separate chassis.
I am saving my MR220 to modify to be a preamp, as it has a great sound to it, but at 17wpc it runs out of gas.