Most educational audition you ever had?

What's your experience? Not what was the best preamp or amp or speaker you heard, but what audition changed your pre-conceived notions of audio in one audition, for better or worse? What was the most educational? Where, when, what, and why.
I've had the pleasure to audition various pieces of audio equipment for the last 25 years or so at dealer showrooms, audio shows, and fellow audiophiles' homes. But for me it was an audition of Revel Studio (the newer version) loudspeakers with all new at the time (500 series ?) Levinson equipment at a dealer's showroom in Northern Virginia, about 2 years ago. I went in only to look at CD/SACD players (from Marantz or Esoteric) but heard this system.
What was educational for me? I found a CD they had that I was familiar with (a Living Stereo or Presence SACD of Gershwin Rhapsody in Blue), and having listened to it in my own system over the years, with changes in homes, rooms, and equipment, what struck me was the soundstage -- something I had never heard before. This soundstage on this older recording, on this system, was confined to a box about 4' by 4' between the speakers, much like a TV. It was like it was cut out with a scapel. The speakers were probably 10' or more apart and I was at least that distance from them. The edges of the 'box' were very distinct - something I have never heard before or since, that is, there was sound from the box and not anywhere outside of it. Almost weird. The dealer did have a good system set up and room, something many of my auditions over the years have not had. So I'm sure that was a big contributor to what I heard. Anyway, for me it was a first, in terms of how distinct a soundstage could be compared to so many other systems and rooms.
So, as to the question of the post, I am not saying it was or wasn't the the 'best' system I ever heard, or the last word in any one category of bass/treble/realism, or musicality, (except soundstanging in my case), but in this way I described, it was very educational for me.
What was yours?
The first time I heard electrostatics in the early 80s. Was shopping for speakers when I happened into a room with Acoustat 2+2s playing. I stood in the sweet spot soaking up the new-to-me sound until a salesperson noticed and proceeded to patiently educate me on the strong points and how to listen. Probably influenced every purchase since.
My epiphany came when I finally got to listen to a pair of Legacy loudspeakers I'd lusted over for several years. I'd gone over them in the mailings I routinely received more times than I could count. Everything seemed perfect: lots of woofer cone area, midrange totally accounted for, and a state of the art tweeter section. The quality of the cabinetry and finish was stellar. And, with a practical and seemingly brilliant engineer, surely the crossover had to be as good as things got as well. With all those drivers, the speaker looked the part, measured (if such a characteristic exists) perfectly, and had loads of testimonials showing how customers found the right speaker after the typical audiophile journey. Who could ever need more in a loudspeaker? It just had to sound perfect. Had to!

While the speaker sported a price I'd not blanch at today, at that point in my life, it was kind of a stretch. But one I planned to make. The mail order might happen any time. Wait, a used pair just came in on trade at one of the big local brick and mortar audio stores (remember them? wonderful places to spend time in). Not quite the veneer I would go with, but a price I could easily swing. OK, my lucky day. Done deal! Since I knew the staff, they asked me to come over, and give them a listen before going through with things. Ah, come on, the decision was already made. All right, all right, all right. Just a formality so I thought, these babies were mine. So, there I was, favorite music in hand, ready to roll. Get ready to throw them in the back of my Bronco, I already folded the back seat down two days ago. They had them set up with a great muscle amplifier. I was ready to ride Space Mountain. Here we go, and uh, um, nothin'. OK, let's try a different CD. No, maybe switch in another amplifier. No? Preamp. Still no good? How about the cabling... I had another speaker do this to me before, but these? My dream, my destination product. NO!!! I mean they left me as disappointed as I'd ever felt in audio.

The (single most important in all the hobby) thing it taught me was that drinking by the label in audio puts one in a very dangerous position. Specs don't tell the story, neither do reviews, or the opinions of others. You NEED to listen to a component, and judge it with the one instrument man has not yet come close to bettering.
the first audition of the quad esl. to this day that speaker comes closest to the real thing.
Mrtennis, how right you are .
One that comes to mind was an all-Rega system at the last San Francisco Stereophile show. It was a modest system that really made music as well as making a big impression on me. There are a lot of audiophile niceties that we get drawn to but I always have to be conscious of the balancing act between getting off on the sound vs. getting off on the music.