In simple words , Denon sounds bit dull(may be flatter) and Marantz sounds bit bright and more separation along with better soundstage(not fatiguing).
Great post, without the typical kind of exaggerated or misleading claim using narrative like night and day difference, even my new born daugher heard the much more musical Marantz sound blablabla..., you nailed it in one simple sentence!!
The thing is, when it comes to subjective measurements, such feedback is all over the place, the only apparent consistency at times would likely be due to internet hearsay and would not be a reliable indicator at all. I have experience with half a dozen Marantz devices including avr, avp, preamp, power amps, and about the same number of Denon AVRs and could never hear different signatures when compared under the same conditions when DSPs are not used. With DSPs, sure there were audible differences but, in such cases, examples: action, science fictions etc.) there would be so many things going on, it is hard to really say which one sounded better.
I really am tired of trying to imagine why so many people could hear differences between
even the virtual twins of D+M units, e.g. Cinema 70 vs AVR-X2800H, SR6015 Vs AVR-X3700H etc. (internal parts and circuitry) when there's precious little differences technically except under one of two conditions when the most sensitive train ears might have reasons to be able to detect a subtle difference. The reason I say something now to such a post is because you happen to be among the few that actually thought Denon sounds dull/flatter and Marantz sounds bit bright..., most (not all..) seemed to have said the opposite. As such, it is a good example of how unreliable subjective measurements/reviews are.
Anyway, if one is ocd enough to search for online comments, one would see a good spectrum of such opinions on different sound signatures of well-designed, price/feature comparable AVRs, some more extreme/or unusual (such as yours) than others, so logically speaking it may be a waste of time to base one's purchase decision on users' opinions on sound quality, compared to simply based one's decision on specs/features and measurements.