Your detailed post makes it easy to respond, and the following are my 2 cents.
- The RX-A1080 does not have better DAC, the ES9007 data sheet is not available on the web, but it is likely the same as the ES9006, just different part number. It appears to have been launched in 2007, along with the ES9006 and ES9008 (a flag ship for years).
- The SR6013 has the AK4458 that actually is 5 dB better in THD and 5 dB worse in Dynamic range than the ES9006, so I guess it is fair to call it even. Both have audio related specs well above certain upstream components that are the real bottleneck so it really is a moot point. Regardless, when you get to this level of DAC specs, it is highly unlikely that the average human can tell any audible difference.
- Both DAC should be capable to handle 384 kHz/24/32bit files if implemented for, and if you can find the contents. Yamaha claimed its RX-A series can do it, but they down sample it anyway, except for pure direct mode. In PD mode, your can only play those high sample rate media from the network and usb input. In practical term, this feature is really meaningless. Still, practical or not, I would give Yamaha's DSP the edge.
- Reviews/musicality etc., those subjective reviews are of little value. Subjective is subjective, there is no point to debate or over think which one is more musical as such. If the reviewer/user claimed he/she could hear a difference, it begs the question whether he/she was comparing apple to apple (sources, speakers, placement accessories etc.), level matched, Placebo, expectation bias. It is extremely difficult to validate such claims, hearsay over the years rule, so even if Denon and Marantz's comparable models are virtually the same internally on the electronics, there is a group of people who would/could hear the so called "warm" sound, regardless..., and that's just one example.
In case you are interested in reading something a little technical, below is link to one good read, that came from a credible (imo) source. If you read the whole thing, you would at least see his point about why the HDAM modules shouldn't make an audible difference, whether you believe it or not..
https://hometheaterhifi.com/technical/technical-reviews/options-by-supplier-and-price/
Between those two, my pick is the SR6013 for the obvious reasons, 5.1.4 is much better than 5.1.2.