The beam tetrode aka kinkless tetrode (hence KT66 etc) is based on an invention by J H Owen Harries who discovered the "critical distance effect". Harries wrote about this new power output valve in Wireless World, August 2nd 1935.
Another important development around that time was negative feedback by Harold Stephen Black.
Before those two developments there were amplifiers that were considered high-fidelty but they used expensive high quality triodes. An important example would be the Wireless World Push-Pull Quality Amplifier of 1934. See Wireless World Quality Amplifiers for a chronology of the Wireless World amplifiers I know of (there are bound to be quite a few others). For a more complete, and very much more readable, history of early hi-fi see A Tiny History of High Fidelity (at www.nutshellhifi.com).
Here are my notes on a Mullard "Five-Ten" restoration