A moment when you realized better sound was possible.

Freshman year of college I heard some DIY speakers some upperclassmen in my dorm had made. Eventually I got to visit a music lab where they had made a pair of subs with 24” drivers. The cabinets were multiple layers of of plywood with sand between the inner and outer box. I think the main dimensions were 8’hx4’wx12’d, but I’m not 100% sure on the depth. I do know they had left the outside as full sheets to simplify the cuts and there was a long heat sink extending from each driver’s voice coil. No idea what the upper range speakers or amplification were, but I’m sure it was massive. I remember one specific test track vividly. It was an outside setting with birdsong, the wind rustling long grass, and a small stream that had lifelike realism. Then way off in the distance was a barely audible whine that slowly grew in volume until it became recognizable as a plane. The plane gets closer and the volume grows until you could literally feel the jetliner flying over your head as it came in for a landing somewhere beyond the speakers. It was surreal. 

Likely to get flamed on this one, but my first "system" as an early teen was based on some no name floor speakers, a Technics integrated from a rack system, double cassette deck, and basic Kenwood TT.  Loved it.  Crushed the heck out of some cassette listening and raided my older sisters' LP collections (AC/DC, Billy Squier, Foreigner, Kiss).

Got an off-brand CD player from a pawn shop, first CD haul was Tom Petty, Billy Joel, BTO.  Hit the play button on that thing, and was blown away.  Not trying to argue that CD is better than tape or vinyl, but on that system with those components it sure as hell was.

I have still been living in mid to upper midfi, but heck I am happy with the way it's going.  With some of the high value proposition components out there these days, starting to feel like I am putting together a legit experience.

iron47 -- You're on your way!  Just try not to go bankrupt too quickly. And try not to make your neighbors too murderous.

Thanks @edcyn!

I have external crossovers. I read on a forum (maybe this one) about how solid core wire sounds different than stranded. I went to the hardware store and grabbed some 20 gauge solid core doorbell wire and ran that between the crossover and the tweeters. This was eye opening for me because it proved to me that cables do in fact matter. 
Two more experiences further opened my mind: upgraded interconnects and a power cable for my amplifier. I began to realize that virtually every piece of the chain is in a sense a component, which sent me on a mission to tweak every last piece of my system to get the best sound possible out of it.