99% of YT reviews are pointless BS fluff jobs.
For the comparable levels of Denon and Marantz product, there is precious little difference between them. These would be more aptly compared by looking at the upper tier of Denon, like the 3700 which is the equivalent of the Marantz SR5015, the 4700 to the 6015...
I may have my numbers crossed slightly with the 3700 comparing more closely with the 6015, but it doesn't make much difference.
At that level, there is almost no difference in audible SQ, when operated within standard parameters (i.e. not being overdriven).
I cannot speak to the Denon you purchased, more than to say that Denon does make a good product and that the differences will be slightly wider than those between Marantz the upper tier of Denon. Is it something you are likely to really hear? Probably not.
Most likely, any equipment differences may be in the Amp stage, DACs and other processing chips utilized, and overall build quality (lighter weight chassis, more plastic, etc).
End of the day, the value of a higher end AVR really lies in the eye of the potential owner and what your goals are. If you want to pursue critical music listening, you may very well find a difference, but would likely also require a more revealing set of speakers. If you want to enjoy movies in your home and you music listening is more passive, you could very well be fine.
If you really want to explore the value of spending an additional 1-2K on an AVR, you need to really determine what your requirements are, what features you need, connectivity options, etc. It may be that in 5 yrs, say, you want to upgrade across the board. This hobby has a way of getting people to do that!

That may well entail you upgrading a new TV, getting the latest in AVR technology, and upgrading Speakers and Subwoofers to something more neutral and accurate... If you chose to consider a path like that, we always recommend starting a little AV piggy bank and setting aside a little at a time until you can afford your next upgrade. shopping smart and upgrading incrementally can get you a long ways in 5yrs time!
In the mean time, if you are happy with the way things are now... ENJOY! Get out when you can and experience some other gear, audition other speakers...
If you want. Only then can you really decide for yourself if there is value in upgrading to more high end gear.
