Several game changing moments for me. Hearing The Dovells' Bristol Stomp on a good quality console stereo in 1963 alerted me to the possibility of deep bass in pop musiic.
A neighbour who built his own stereo was an eye/ear opening experience. He bought the cartridge and motor and main bearing, made the platter, arm and all the rest of the turntable (thread drive from the outboard motor),made the cabinet (thin woven wood for ventilation) made the preamp/amp (Williamson) mounted two 15" drivers in a clothes closet (clothes were damping) two 12s hidden in an amazing decorative wall for midrange and multicellullar horn tweeter sitting on top of the closet firing at the high cathedral ceiling for highs. This was mono, but it didn't matter—it sounded spectacular. The guy was amazing—he built a photo enlarger, including bellows from the lenses alone. The most influential people I knew for about 20 years. Built three beautiful houses, all the furniture, a guitar and a balalaika just for fun.
Much later it was game changing to finally realizie amplifiers make more of a difference than I suspected after comparing my NAD with a Mission Cyrus with 1/3rd the rated power more usable dynamic range. Also a Sony receiver after being familiar with a Quad 33 on the same system. Ouch.