If you look back through the history of high end audio amplification you see numerous design trends, followed by patterns of development to address their shortcomings and maximize their strengths. Take vacuum tubes for example. The first power amplifiers were what we would now call SET (single ended triodes) where one tube fed an output transformer to power a loudspeaker. This worked great when speakers were of very high efficiency and not more than a few watts were needed.
As demands for higher fidelity and greater frequency extensions were demanded by the growing number of hifi buffs, more power tubes and larger output transformers were added to build higher wattage amplifiers that could drive less efficient loudspeakers. Then the weakness of transformer coupled outputs became more apparent as the loudspeaker loads got harder to drive. To address this issue, one camp went off in the direction of removing the output transformer altogether (OTL designs), while another marched forward with even bigger transformers and massive numbers of output tubes. I can remember seeing some Sonic Frontier, Audio Research and Jadis power amplifiers that looked like miniature glowing cities and probably weighed close to that of a building. Extreme approaches were de rigueur for the time.
And then along came solid state amplifiers and we began an entirely new chapter in our quest for music. Some of the first transistor amplifiers even had output transformers mimicking that of a vacuum tube power amp. We'll take a closer look tomorrow.