Study: Evolution of the commercial amplifier

Hi all. I'm trying to better understand the changes that Marantz has made to their same basic symmetric design that they've used for over 20 years. What has actually improved, and what were just manufacturing decisions based on the parts available and power efficiency?

Note that this not an HDAM discussion which IMO was just their injection of proprietary IP into an amp that I can otherwise build myself.

PM14 - 1998

SM6100/PM8200 - 1999

PM6003 - 2009

PM6005 - 2013

PM6006 (AND PM6007 AFAIK) - 2016-2020

So what exactly have they done that's good?
  • Eliminated most diodes from the signal path? Is this to reduce noise or is it to save power from Zener leakage?
  • Added DC offset control? Is this for idle power savings as well?
  • Added series transistors (Q6007-Q6011) in the input stage? Why?
  • Switched from a darlington to a regular high-gain bias transistor?

I ask because I wonder what I can learn to improve my own symmetrical builds. Why do even their latest flagship models not thermally couple each transistor stage?

Many thanks
I suspect they added the series transistors to lighten the load on the first stage to improve the gain as a way of compensating for the removal of the input amplifier/buffer. The change of the vbe multiplier may just be because of different output transistors and/or possibly the way they are mounted on the heatsink.