The "marantz tax", "Pioneer tax", "JBL tax" et al is very simple, it's a collector premium. As such, the more desirable it is in their line and the further above average condition it becomes more collectible and the premium increases. I have seen pristine collectible models of vintage audio sell for ten times the selling price of rough ones, I have sold pristine examples from my collection for ten times what people are getting for rough ones, because I only collect pristine ones I know the difference.
If it's in beat condition and a mid-to-lower end model it's affordable, ... if one of the top models new in box it'll be rediculously expensive. I don't know why people have such a hard time grasping this concept but it's pretty much the same as trying to compare a rare and autographed baseball card to the value of the paper on which it is printed, ...
The value of vintage marantz (etc.) receivers is determined by the people who are buying them and nothing else.