Hard to say for sure, as I have read the schematics of the 2017 models only. They included the AVR-X4400H, SR6012 and SR7012, also the AV8805 that had been available for download occasionally (no idea why Marantz would do that).
I am quite sure there has not been changes in the preout from 2017 to the 2018 (based on what I saw on the AV8805) models though. So based on the 2017, and again, quite possible the same applies to the 2018 models, the one step down X3600H, SR6013/6014 should perform basically the same as the X4500H and SR7013. The upper models may have a slight edge on the noise part of the THD+N because of their nicer layout, shielding schemes etc.
The interesting about the HDAM vs no HDAM is that in theory HDAM should help at higher output level, say above 2 V, and/or when used with external amps that have lower than average input impedance, but having that extra buffer stage with as much as 10 transistors would also likely add distortions. I was wondering about the latter, than I read Dr. Rich's review on the AV8805 and apparently made some comments that seemed to line up with my own line of logical thinking.. Regardless, I can tell you that based on my experience with the X3400H and X4400H, those are fantastic values and sounded as good as my higher end separates, including real separates, and the higher end AV8801, so no, I do not believe HDAM made any damn difference to me audibly speaking.
If you go by measurements, I don't see any meaningful difference between the Denon 3000/4000 and Marantz 6000/7000 series either. To me, if you can wait, you should be able to grab a SR7013 for US$1199 (been there in Canada 3X this year (and it was Canadian $1199, ie.much lower in US$), or $999 for an AVR-X4500H.
If you live near the Canadian border though, then nothing beats the C$798 AVR-X3600H, then you can use the savings to grab a 2/3 channel power amp.