More food for thought re your question on whether HDAM adds intentional distortions for the intended "marantz sound".
You mentioned you have a PM6006 with THD+N spec: 0.08%. That's at the power amp output, at pre amp output, where HDAM has effects, for sure it would be lower, likely less than 0.005%. It is 0.005% for the SR6014, I expect the PM6006 can do better.
Did you know their flag ship integrated amps were 0.005%, and that's at the power amp output so the pre out's would likely be in the order of 0.001%!!
Those integrated amps have the much higher end HDAMs and are situated in the right place, that is upstream, instead of at the power amp input like they are in the AVRs and AVPs.
At 0.005% distortion/noise level, you can rule out the effects of any "intentionally added distortions", how could it be, if they did add distortions then why would be so low? What would it be if no distortions were added, 0.002%? Can we hear the difference between 0.005% and 0.002%, I guess not.
By trying to answer your question, I am now fully convinced, by logic alone, that while HDAM would in fact add a little bit of THD+N. but no it wouldn't be intentionally done... It adds distortions and noise because that's what electronic amplifiers do, HDAMs are unity gain buffer amp stage so with that extra stage, you get extra distortions and noise. As to why would they do it that way, with the HDAM, I have an idea but I would rather keep it to myself this time. May be bigguyca wouldn't mind telling us what he thinks the real reasons may be...
Yes i see what you mean that there is no way that distortion is intentional, as than all their higher end models would also carry the same amount of distortion to have that "Marantz" sound. My other point/possibility though was that the HDAM does something desirable (to their ears anyways since they claim to tune by ear) to the signal, but in the process adds distortion aswell shown in the measurements. They talk about the benefits of HDAM in their website which im sure you've read, however many of their claimed benefits are hard to measure without taking the amp apart and spending a ton more time. Noise and a low slew rate for example they claim is one benefit, the SNR of the PM6006 is 102DB which i though was pretty high for its price range. Or here:
"This results in lower noise and an improved slew rate for richer, more dynamic sound that accurately reproduces slight high-frequency details."
The statement from their website above is very subjective; im not sure how one would go about measuring "accurate reproduction" and "dynamic sound". What sounds natural or accurate reproduction of high frequencies to my ears may be different than someone else. Some people like paper cone woofers, others aluminum ,some like silk tweeters others metal.
Source:
https://www.marantz.com/en-gb/blog/...mic-amplifiers-modules-do-for-your-home-audio
I have to believe it was a tradeoff to the marantz engineers/listeners to integrate their HDAM technology at the expense of added distortion since we are arguable dealing with inaudible amount distortion being inadvertently added. If not than i just bought a MSRP $950 CAD amp that performs like a $100 SMSL A18, lol...at least mine has a cool analogue volume pot that uses a motor when you adjust the volume with the remote....its pretty cool