Even in 1980 they were realizing possibly the biggest advancement in upgrades of this audio technology was not the exact topologies but the capacitors it seems.. I have heard myself bigger sound differences using the same component with different caps over buying other totally different audio in many cases.. Interesting article below, check out the date!
http://www.reliablecapacitors.com/pickcap.htm
http://www.reliablecapacitors.com/pickcap.htm