These two valves were originally designed for RF frequency changer applications at 220 MHz in TVs etc. They were also used for video amplifiers with the triode as cathode follower from the pentode to lower output impedance at RF or Video.
The design of HiFi and industrial feedback amplifiers usually involved applying feedback over the entire amplifier - but problems arise with uncontrolled phase shifts when pentode amplifier stage is coupled to a triode stage - triodes have variable Miller capacitance from anode back to grid and hence the input impedance is not closely controlled.
In industry I used triodes as cathode followers to give low output impedance from the pentode stage, with any phase correction components on the cathode follower output with more controlled results. However this necessitates another valve in the line up - not so important in industrial applications where performance was the priority, but in commercial audio applications cost was the controlling factor.
Hence Quad used another pentode as the second stage rather than the conventional triode. Dr Bailey spotted that an ECF82 could be used with the pentode section offering high input impedance to the preceding pentode stage, with the triode section being cathode coupled as a long-tail pair to give the required push-pull outputs to the final stages.
I think he was working as advisor to Arthur Radford at that time, and hence the new audio amplifier with more, better controlled, feedback came on the market. The final hurdle was the output transformer which again was specially developed by them to control the loop response.
I have built two of these amplifiers for my own HiFi system (but sadly one of the output transformers has failed and I can't find who manufactured them (it may even have been done in house by Arthur Radford), and tell them how important their ideas were to me when designing the 6ZC amplifier for the MOD (where I used cathode followers to control the loop response so that the unheard of 60dB of feedback could be applied with adequate Phase and Gain margins for equipment to go into production and remain stable over life in shipboard applications.
I was praised by the MOD officials for such a good stable design.
See also New Phase Splitter