The story
The company initially focussed on provision of amplifiers for public address systems and theatres, with only a few staff. Typical designs used were similar to those found in the Partridge Public Address Manuals of the time, which used Osram's DA30 power triodes in push-pull for 45 watts output.
During the latter part of the Second World War, the company started developing amplifiers that made extensive use of negative feedback to achieve very high performance. This approach had been patented by Harold Black of Bell Telephone Laboratories in 1934, and already used before that by Alan Blumlein of EMI, but it was slow to emerge as popular method for controlling amplifier performance.
From Wikipedia’s article on LEAK, shared under CC BY-SA 4.0.3