I wanted to express my view to this forum, as this is an interesting topic. I will honestly say that my evaluation of a receiver is simply to play music that I like that can reveal the capability of an amp / receiver with the source material. I've owned both Denon and Marantz over time; both have sounded good to me in the time frame I owned them.. From a measurement perspective, I can't evaluate, so this is my subjective perspective.
Denon was really the brand that brought me into higher-end audio with their 5800 many years ago. Over time, I have owned both Denon and Marantz with my last receiver being the Marantz SR7012.
Recently, I had a lightning strike my house, and it took out my Marantz SR7012 receiver along with a lot of other equipment (the amps in my Triton Reference also were replaced). So, I was brought back into the market rather abruptly. I read quite a few reviews on both Denon and Marantz current receivers and decided to take the X4800H for a test drive first; primarily because of the perceived value vs $ spent.
I have found that the best listening test for any audio system is live / two microphone recordings such as Miles Davis - Kind of Blue and/or Harry James 1976 & 1979 recording sessions by Sheffield Labs. The sound stage of these recordings is challenging for all but the best equipment and systems; so it is a great platform to evaluate equipment. When evaluating equipment this way, I find it easy to compare brand to brand or model to model using these recordings; the differences aren't subtle.
The rest of my system: Gold Ear Triton Reference, Primare A30.5 5 channel amp, Oppo UDP-203 disc transport.
My first test was to listen to the X4800H using it's built-in amplifiers. On Harry James recording, what I found was a very muddy sound stage. The sound field of the cymbals on the drums did not "rise" on the back of the stage. The piano is muted along with the base Cello. Instrument position on the stage was not precise. I wasn't sitting in the church with Harry James and his band. My older Marantz SR7012 sounded much better to me under the same conditions. I then connected my Primare amplifier, and listened to the same tracks. Much better, although there was still a lack of vibrancy and a sound stage that was .....muddy.... and not revealing what the recording was fully capable of reproducing. I had read about the AKM Fire which caused the DAC changes in the X4800H vs SR7012. I was starting to think that I was hearing these differences when using X4800H as preamp. I returned the X4800H. I decided to buy the Marantz Cinema 30 vs the Cinema 40 to get the better DACs and hopefully better amplifiers. I connected the Cinema 30 as preamp with the Primare for this test and played the same Harry James recordings. Finally, the vibrancy I was looking for with accurate stage placement had returned from my SR7012. The Cinema 30, I believe, sounds a step above the SR7012; as it should. But the X4800H sounded a good step below both Marantz receivers on the source material I listened to. Here is reality: I wanted the Denon X4800H to work out because of the lower price. However in my listening tests, it just didn't stand up to my older SR7012, little less than the Cinema 30.
I haven't tested the Cinema 30 amplifiers yet. That is to come. I'm hoping for a much better experience than the built-in amps of the X4800H.
So, is Marantz dead? I don't think so. In the discussions on this thread, other posts mention the Marantz sound, warm sound, coloring, etc. I'm not sure that I'm hearing this. I did play around with both DAC filters and really did not hear a difference. However, the Cinema 30 and my previous SR7012 both gave me a more pronounced feeling of "being there" vs my listening test on the X4800H. Was this difference the AKM vs TI Dacs, something else? HDAM? Maybe the X6800H would have made a better more equivalent test. However, the difference in costs versus the better amplifier section of the Cinema 30 was the driving factor for choosing the Cinema 30 vs X6800H.