Kenwood in the 1970s spent a lot of time and effort studying it and the results were DC-daylight amplifiers with ultra short, heavily braided/woven special cables and monoblocks placed right next to the speakers. They also pursued 'Sigma Drive' which essentially put the speaker cable in the feedback loop.
Yamaha pursued 'Active Servo Technology' (AST) which put the speaker in the feedback loop and lifted it from ground, thereby essentially creating a 'negative impedance' drive and also employed cartridges (like an Atari game cartridge) that compensated for the impedance/phase characteristics of the attached speakers. Each 'AST' speaker had a basic crossover and the combination of the individual supplied cartridge and the 'negative impedance' drive gave us small speakers that performed incredibly well. Integrated amplifiers with AST came with a 'flat' cartridge for normal speakers.
Sadly, AST died in the marketplace in about 18 months or less.