There is a bit more into in the Rega book 'A Vibration Measuring Machine' which I would highly recommend for any Rega fan.
The circuit is based around a John Linsley Hood design which tried to get close to Class A performance without the heat penalty. Hard to get working at the time (c1970) as the design requires close physical and thermal coupling of drivers, thermal tracking diodes and output devices. Today using surface mount tech and Sanken output devices which integrate some of the parts (look closely at the IO output transistors and you'll see 5 legs instead of 3).... well it now works. Brio is the same and so I believe is the Elex R and Elicit R.
Ah. I searched a bit online and found the circuit in question:
https://startfetch.com/jlh/jlh-1970-ab-amplifier.pdf