On a unit like your Marantz, I'd use a medium heat iron, say 30 ish watts, with a fresh clean tip. Also top quality solder removal braid. Techspray is one such brand. Having worked on Marantz of this age, I know you'll be replacing every last electrolytic cap in the unit. I find the old ones failing to the point corrosive goo is leaking out and in many cases, dissolving their leads.
Marantz of that period are fine sounding hi fi specimens. You don't want to wreck one by learning how to use a big funky old solder sucker on it. Try that thing out on an old point-to-point amp first, to learn what it will and won't do. I'm worried you'll burn the traces off the Marantz's circuit boards.