Hearing electrostatic speakers for the first and second time. First time the RTR tweeters in the ESS Transtatic I, the second time the Infinity Servo-Static I.
Then it was hearing a direct-to-disk LP played by a Decca cartridge. I don't separate the two because their sound is so related. Lightning fast transient "snap" (like the crack of a bullwhip), attack, immediacy and tactile presence, punchy bass (when played aggressively, the strings of an upright bass, piano, and bass drum really "pop"), and the startling dynamics of live music, Vocalists sound spookily in the room, standing right in front of you.
Then it was finally hearing a loudspeaker that reproduced the size and scale of instruments and ensembles: the Magneplanar Tympani. They make just about all box designs sound like the instruments and vocalists are being squeezed through two missing bricks in a wall (left and right), with the vocalists mouth about three feet off the ground. Pianos through them sound like miniature toys compared with big planars and the real thing.
Those are things I heard without owning them, not what I did to a system. But then I acquired them, so I guess that counts.