I've just been through this, albeit on the 4500 and 6014/7013. My thoughts:
- Started with the Denon X4500 due to good experience with older X2200, running with KEF Q speakers (since changed to Rs). All seemed fine to me. As I continued to shop and add components, multiple demos in Best Buy put the 4500 against Marantz and Arcam models. I understand Best Buy Magnolia is not always ideal, but it was better than nothing. The Arcam was in a league of its own but did not appeal to me due to other features. On blind testing with multiple family members and associates, everyone thought the Marantz sounded better. There was simply more life to it, brighter, warmer, more energy if you will. It made the Denon sound flat with KEFs (which are neutral speakers)
- I left with the Marantz 6014 but had Atmos issues connecting to Xbox One. That aside, it sounded better than the Denon. Even my sub had more life. Ended up swapping the 6014 for a 7013 which cured the Atmos issue (assuming more stable firmware) and simply sounds fantastic
- I am not a fan of the Marantz porthole, and wanted to use Denon. Top down, the 4500 and 6014 looked identical (even looking inside). The 7013 is larger, but looks similar to 4500 and does share the same power consumption (710W).
To your questions
- I'm no expert but comparing the 6014 to the 4500, no. My room is 18x15 and once calibrated I never needed that much headroom. Now, as I expand speakers that may be a different conversation (current 5.1, going to 5.2.4)
- Menu's are identical - black and gold on Marantz vs. black and blue on Denon. Build quality feels near identical also - they share so much (remotes, cables, components/internals from visual). I thought with the 7013 it would be made in Japan, but the box says made in Vietnam 9 (as was the 4500, probably by the same team)
- I can't speak to Klipsch, but across multiple speakers in test, and my preferred KEFs, Marantz simply sounded better. People will comment that for music it is better and no difference on movies or Denon has the edge, but I disagree
- My research suggested the same on model years; I just like current versions, although I somewhat rolled back with the 7013
- If warranty gives you peace of mind then 3x coverage is important
I don't like looking at the Marantz, the porthole is useless to me, and I wanted the Denon. But my ears won, and luckily I watch the TV, not the receiver.