Help needed with Linn LK2 schematic

[lce]

As an owner of a Linn Lk2, I try to understand how it works.
Thanks to this very informative thread :
tweaking classic Linn LK1 & LK2 / LK280

I got the schematic, put it under LTSpice and now try to understand it ....
As it is not really simple and as I'm not really an expert, I need some help to understand the design choices ...

1) The input stage is a classical differential pair , with constant current source and current mirror.
I just wonder why they use 2 transistors (Q5,Q6) linked by R3 for CCS instead a more classic one transistor one..

2) VAS stage is another differential pair, cascoded input, CCS and current mirror.
Don't really understand the bias scheme (zener D1//(D2,D3,D4)) and why they don't link Q2 collector output
to Q8 base input in a more symmetrical scheme...

3) I must say, that I just don't understand at all the third stage (Q15-Q17). Seems to be a complex Vbe multiplier ,
but why so complex and why it is linked to amp output ?

4) Output stage seems to be a CFP with 2 current mirrors, but why the 2 output diodes D12,D13 ?

All this (26 transistors !) feed by a fully regulated power supply ! (when I say not really simple ...)

So lots of questions and some more :

I plan to recap it because of its venerable age.
Any idea about which cap for each function ?
Without changing all the topology, is there some gain to change some components (better output transistor,etc ... ?)

[lce]

Really nobody to discuss about this very "interesting" schematic. ?

[lumanauw]

Do you have the sim result of open loop gain curve for this amp?