For me, it has to be my introduction into tubes circa 1989, with a pair of Quicksilver 8417 mono-blocks, because shortly there afterwards the supply of power tubes started dying up, and at the time I didn't know jack about how to find more or even what brand were in them - G.E. of course.
But the sound of these heard 6 years through a pair of ProAc Response One S at the friend home I sold them had me wanting to throw myself from an 18 story building, since then I've learned four very important things - live with any given component long enough to understand its strengths and weaknesses - find the right combination of tubes to enhance the sound - a cabling loom that matches - and isolation devices/platform to lock in its sound.
Life's lessons indeed.

h