An unsolicited suggestion (n.b. I am not a moderator): don't just post the same text in several threads, it appears for all practical purposes like "spamming." Instead, post a question specific to that thread, even if it means rewriting a bit. It's not what you ask, but how you do it.... I'll watch out what I ask in future. I figured getting point of views and advice from folks that might have owned both brands, or even one, can guide well, considering they're all experienced in these things.
I hope that's all that needs to be said. Now, back to the RX-A discussions. Comparing the Denon to the Yamaha, both are probably fine AVRs. The claim that one brand sounds "warmer" than another is IMHO poppycock. It really comes down to features. I haven't the slightest idea how the X6400H compares to the RX-A3070, but the RX-A series is generally a bit higher quality than Yamaha's RX-V/HTR/TSR lines. You can compare their product pages online to see if one has something you NEED, or you can plug each of them in for a week and see how you like the interfaces.
I can tell you that Yamaha's AV Controller app for iOS and Android is one of the better hi-fi manufacturer bits of software I've used, and they regularly update capabilities for even my 2012 AVR. Compare with someone like Samsung, which actually dropped support for my TV after 3 years so their app was useless to me.
The Denon features Audyssey speaker setup and calibration, there are some who live and die by that app versus Yahama's YPAO. Running each for several days with speaker/room correction active (Yamaha calls theirs PEQ) may tip the scales for you. But definitely run both in un-corrected, "straight" modes as well.