Hi Rupert
The 306 and 606 are essentially the same circuit other than final power output,.
There are some very minor differences suggesting the 306 was used to evolve the 606,
and the 306 was still evolving. For instance c4 180pf in the 306 but missing in other
schematics. R8 available on the pcb of the 306 but not allocated or mentioned
in schematics. R42 on the 606 allocated differently in the 306 as R35

The interesting section for me is CR2 a Vishay fet current regulator called a J503
I think Quad selected this part, principally because it was a 2 terminal regulator
and would work with pcb layout. Its output current is very low indeed, possibly too low
at below 1 ma in fact 0.56ma , asking if it is sufficient for both T1 emitter and T3 base combined.
T3 base certainly, but T1 emitter as well ? It suggests a second CR2 for the emitter
leaving the existing one for the base might provide better result. A LM317 will
work here, but messes slightly with turn on. If I find a better way of doing this
I will post it in the DIY section. Also arranging all of the 6v8 as 7v5 and instead
have these with transistors rather than just zeners will bring great result..

Agreed though the 306 is a transparent amp, one of the best Quad ever made

Cheers / Chris