What was the first system you heard that made you think the high end made sense

I am sure I cannot be the only person here who can recall the day, and indeed the very system that made them think 'oooohhhhh yesssss - I get it - I understand - and I absolutely love it!'

In my case I attended a HiFi Show in Heathrow at the Renaissance Hotel. It was the GT Audio room hosted by Graham Tricker. The parts comprised:

DPS turntable- Schroeder arm - jan allaerts cartridge
Tron Pre-amp
Avante Garde Uno's

In all honesty I still remember the day now. It was so musical and had the ability to connect me to the music. When I went for a full demo at Grahams home I came to understand the magic of horns, valves and imaging. 

The first time I ever heard an SET amp was at a SET Symposium in Philadelphia put on by Stereophile magazine.  A whose who of audio people were there and that day I got to hear about 10 different SET amps using 845s, 211s, 300Bs and 2A3s all played thru a CAT SL1 preamp and Sony SACD player.  To me, the best sounding amp was the lowest powered and least expensive as well, Fi 2A3 monoblocks.  That's where I took my username because I think SETs sound best.
Big Krell amplifiers and Martin Logan Statement speakers. I can't recall the other components.
1978 Klipsch setup of some sort coincidentally at G&T (Groove and Tube) dealer in Lancaster PA playing Fleetwood Mac Fleetwood Mac album.    They also had a Magnepan Tympani setup that also impressed me.  I never heard the term hi end until many years later possibly first on this very site. 
Panasonic boombox. I thought it sounded so bad that to get a good sound I would have to have a high-end set-up.
ESS Transtatic speakers driven by an ARC SP-2C pre-amp and Dual 50 power amp, the source a Sony turntable with a Decca Blue cartridge mounted on the Decca International arm, in 1971. Pure lust. I heard the original Infinity Servo-Statics shortly thereafter, and starting saving. By the time I had the $ the following year, the Magneplanar Tympani-I's had made it to the West Coast, and I got myself my first serious system.
I had B&W 805 first generation. Around  year 1991
I hooked up  Krell KRC preamp and a Krell 200 watt amp I purchased used,
and I thought I had new speakers.  

Quad Esl 57, heard for the first time around 1970. I bought a Quad 33-303 amplifier the next year, after my high school exam, and a pair of Esl 57's a few year later again when I started graduate work and had the space to house them.
Spectro Acoustics Amp,Preamp, Equalizer and Tuner with Clarke Systems speakers a J A Mitchell Focus One Turntable bought the system that day. Added a Revox B 77 Reel to Reel. Nice system for 1980.
It was 1975. My neighbors system:
Philips 312 electronic TT
Shure cartridge
Kenwood receiver
Bozak speakers
All for $1300.

Many years ago (35?) I heard a pair of Apogee Divas being driven by Krell amplification at a HiFi News show (London, Heathrow I believe). That is when the high end really made an impression on me. I still dream about owner a restored pair of Apogee Divas one day.
1976.  Had saved enough summer job money to buy my first "stereo":  Panasonic "Thrusters" system.  After returning it for exchange three (!) times because of unstable pitch from the turntable and which the salesman could not hear, the poor guy said "kid, take your money back and go somewhere else".  Went to Sound Components in Coral Gables, Fla.  Had never heard high end equipment before and will never forget the experience of hearing stacked Quads/Janis subs/Sequerra ribbon tweeters driven by Levinson ML-2's with Levinson pre and Linn table.  My jaw dropped!  That was the day the sickness began.  
Klipschorns at Hal's Hi Fi in Trenton, NJ around 1976. I don't know what the rest of the system was. I bought them.
I guess, besides having been addicted to car audio upgrades, it was when I first heard the Spica TC-50s from an AR tt, listening to "Let's Dance" lp.

I still have a pair of TC-50is in my closet.
1974 my first system a Lafayette receiver,, Klipsch Heresey 
That came bare wood I hand stained and French waxed 
Nakamichi 3 head cassette deck ,and AQ  wire .I was stoked.
It was great mostly  Made in U.S.A.,The Nackamichi cassette deck 
From Japan- great quality their Dragon deck still a classic.
It was when I finished my first summer job and decided to splurge and buy ATH-M50Xs. I'd only listened to cheap earbuds all my life, but when I could actually pick out where instruments and voices were, it got me ~very~ interested.
September 1973, Palo Alto. Goodness, but as a group we do seem to be getting on, don't we?

Magnepan Tympani 1a's with ARC electronics and SP10, playing Cat Steven's Tea for the Tillerman (Wild World).

Yes, we are getting on. 1974, Infinity 2000 Speakers in rosewood, Mac C-26, Crown DC300A, Dual TT, Shure V15, Yamaha tuner, Sony R to R. Our local dealer also used Cat Stevens when demoing speakers.
Hey Terry, I know where you heard that ARC/Tympani set up in Palo Alto: the first high end shop I discovered and visited in '71, Sound Systems. I heard the ESS Transtatics ($1200/pr) there, as well as Infinity 2000A’s ($600/pr), but couldn’t afford them. So I got a pair of the 2000A’s little brother, the 1001. Had them until I got Tympani-I’s the following year. Amazingly, I now own pairs of both the Transtatics and Tympanis, though they are the T-IVa, not T-I.
Churchill Audio in NYC around 1977.  I had been using a SONY HP-161 all in one turntable/ radio compact.  I walked in to buy a pair of Pioneer headphones and the salesman plugged them in to a top of the line Technics FM receiver and the song playing was the Beatles' Yesterday.  I had never heard anything so magnificent.

At a cousin's home. 1969. He had a Marantz 2230 receiver, KLH 6 speakers, and a Garrard TT with Shure cart. The clarity and bass really caught my ear. I loved music but wasn't quite to audiophile status back then.

I bought my Infinity 2000A's at Sound Systems in Palo Alto. They was the first high end shop for me also. I lived in Redwood City at the time where you could by a nice house for $32,000, now $1.5 million, same house.
Yes, Sound Systems. Nice people.

Damn near bought the T-IV's, then T-IVa's. Each time was side-tracked with something else - mostly losing investments. Now it's different - I don't invest in anything but immediate gratification. Hence DIY. And Quads, of course.

Oracle Delphi Mk1-Rega rb600-VDH mc10

Cary Slp 94 (slightly moded with AudioNote copper caps)

Cary SLM 100

ProAc Response 2.5

Siltech Wire.

this system was so musical and inviting I fell I love with music all over again. and i'm assuming thats why we do this in the first place. 

 Of course you just want more so the quest begins.

Actually hearing SET amps was the 2nd time high end made sense.

The 1st time was when I wandered into Chestnut Hill Audio in Old City, Philadelphia and heard little concrete speakers, Rauna Tyrs thru a 70 wpc B&K ST140 amp. Eye opening! Way better than the Pyramid Met 7’s that I had listened to for a long time in other stores and always coveted
Houston, 1982, Audio Concepts.
 Linn LP-12, Mark Levinson, Magnepans.
Unbelievably wonderful sound.

Actually when I put together my first seperates, all adcom, amps, cd, processr and Polk srs sda 3.1TL. 1984 man I was in heaven. Since then I've come a long way and invested way too much in trial and error. Only remaining pieces are the Polk in my HT system and 1 adcom 555II as spare, just in case.
In 1971 I was only 11 and had a garage sale to raise money for a better stereo. $320 later I got a Pioneer receiver and a pair of Rectilinear 10B speakers from Tech Hi-Fi. Now I'm not saying they were high end, but at age11 they were the coolest. The weird thing is while my current system blows the original away, my memory tells me I was more impressed with my first one.
I was with a coworker who needed to stop by his mothers house while we where out. His step father was a modest fellow who drove a Lays Chips delivery truck, the house was a typical modest middle class home. While there my coworker asked if we could listen to his step dads set up. His space was a 12x12 room with a sofa toward the rear at center but well off the rear wall, with obvious room treatments. He was running Magnepans, a single two channel amp, a very basic pre, basically a volume controller, a CD player, and a 8inch bass cube. He mentioned he was running everything to follow the shortest path from the CD player to the speakers using only what was absolutely needed. He played Dire Straits Brother in Arms, it was fantastic. I immediately questioned the system I had at the time, a Best Buy Pioneer AV receiver with low end Klipsch towers. I got home fired it up and literally said to myself (this sounds like bad) and thus that’s where it started, Damn that guy.

Summer 1970 at my friend's house. A pair of Large Advents and a Kenwood integrated amp. Thorens 125 TT with Shure cartridge. Listened to Hendrix Electric Ladyland, Its A Beautiful Day, Procul Harum A Salty Dog. Audio bliss! Enhanced by taking acid!
Winter of 1970. Shacked up at a girl's house for a couple of weeks and she had in her living room a tube Fisher receiver and a pair of KLH speaker's. The sound had a very silky ultra smooth midrange that would stop you dead in your tracks. That got me hooked. One thing I will never get is this. When Henry Kloss left KLH and started Advent in 1968, his Advent design's were inferior to his KLH model's, especially the three way KLH model with its electrostatic tweeter that was the best model, and best sounding speaker him and his design team ever produced from 1964. I had a large pair of Advent's from 1976, which had much better bass slam than his previous KLH model's but the mid-range and high frequencies were no match against KLH. The Advent's were harder sounding and lacked the top end smoothness of KLH.
Marantz tuner, Garrard TT, and Superscope speakers circa 1977. My very first and favorite set-up. Wish I still had it. 
My late friend Courtney Carroll got me into audio way back in 1979-1980. He sat down with me and before listening for the first time we smoked some Colombian and it was on! I still remember that system. He had an ARC preamp and amplifier, Tympani 1D’s panels and using Mogami speaker cable. I have been into audio since that experience. ;)

My pivotal experience was as a child in the late 50s, listening to my parents 78 records on a monophonic console system with built in phonograph and radio of course. 

I would lie on the floor with my head centered under the only speaker and listen to record after record.  I didn't become a gear junkie until I started earning money and reading the mail order supply mags.  I've been upgrading that experience ever since only to be basically "done" now as I hit retirement. 

The systems I bought started in late high school, followed by a number of incremental changes over the span of many decades.  My current system starting from scratch once again, has taken about 6 years to sound the way I've always wanted.  Barring component failure or my life cut short, this should be my longest held system.  I seek nothing further at this point other than adding to my music collection.