Actually I have been speed reading service manuals that are available on the internet and to my surprise I found many improvements but not too many reduction of "over-design" as such. D&M and Onkyo's tend to get lighter, but it obviously has a lot to do with technological advance, such as using SMTP where possible instead of linear power supplies. Even the class AB amps equipped AVRs now use SMTP everywhere else except for the power amp section. Transformers could be wound tighter, using cores that have higher power to weight/volume ratio, fans+heat sinks vs just heat sinks, more plastic etc.can all contribute to the lower weights. I compared the specs of the output devices and found no reduction there either. In fact iirc the 11 channel AVR-X6400H uses the same output devices as the ones in the much heavier 7 channel AVR-3805.
We already know D&M have already upgraded the DAC, multi-plexors, and preamp/vol control ICs. On top of that, someone knowledgeable and obviously have access to more service manuals than I reported that the newer unit, definitely the current models have even upgraded the tolerances of things like resistors, form the typical 5% to the thin film type that have 1% tolerance, amazingly without any fanfare. Also, Marantz has upgraded the HDAM modules in major ways (for example, 10 transistors vs 6) since the 2016 model year, probably prompted by Dr. Rich's criticism on the previous version.
I have no access to the service manuals of the newer Yamaha models but it is my reasonable assumption that they must have been upgrading too. Their weight go the opposite weight, they have become heavier than comparable D&M units since around 2013/14, though their heat sinks (non flag ship models) look flimsier. I don't think they have build in fans and their enclosures are probably more heavily built, even have an extra support at the bottom for the top models.
It is hard to believe they (at least D&M) can do it while losing money, but that's what the service manuals show.
