For me it was the introduction of the Compact Disc. That was an unprecedented game changer. All the years spent chasing down perfection with a flawed medium (tape/LP) was thrown out the window.
All of a sudden, more power was needed. Quieter amplification. Ruler flat response gear and speakers that could take the phenomenal dynamic range of digital without vaporizing.
I still feel incredibly lucky that we enjoyed the transition to digital and benefited from it. Very little has happened in the last 35 years that is "game changing" like that. DSP is touted, but we had it in 1985* and apart from the fun you can have with it, it is not a universal panacea.
EDIT: I am referring to the incredible Yamaha DSP-1. I have several in my collection and it comes extremely close to "game changing". No other component has wowed me the way it did (and still does if I pull it out just for fun). Yamaha created the original six channel DSP to enable concert piano players to experience what their pianos would sound like in world famous venues. We got the consumer (pro) version to play with.
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