4 first times stick out (3 with guitar, the last one with bass)
1) in a music store decades ago when I was still pretty new to electric instruments, I plugged some guitar (maybe mine) into a Fender Super 60 1x12 tube combo (Fender’s early 90s take on getting a Marshall sound) at a local store. After having only played through solid state guitar amps till that point, I was immediately and dramatically struck by how I suddenly sounded like a Who record. That’s what I’d been missing (as opposed to using Boss overdrive and distortion pedals through my solid state amps). Shoulda bought it but it was out of my price range. I later bought a used rack mount version of the Suoer 60 with matching closed back 1x10, and had a lot of fun with it, but never felt like it gave me "that sound" (maybe due to the different speaker and cab? I dunno — the amp w/ cab is now long gone).
2) a decade later, in a vintage music store in Asia, I found a beat up old Strat and plugged it into a silverface Pro Reveb. Holy cow. My guitar playing buddy was with me, and both our jaws dropped. I sounded just like Stevie Ray Vaughan.
That was the tone. Kinda should have bought it, but a) I’m much more of a Les Paul guy than a Strat guy, b) I like Stevie Ray a lot, but it’s not like he’s my favorite player or that I especially aspire to sound like him, c) it was hella expensive for both the guitar and amp, and d) I was afraid of buying such an old beat up piece of wood (would it fall apart? I’d never bought anything like that and didn’t know). I didn’t buy it, and writing the reasons out now, I shouldn’t have, but I still think about that one sometimes. On the other hand, it would have been a great financial investment.
3) Same store, same amp and same friend around a year later, I found an old maple neck Tele and plugged it into that Pro Reverb amp and… oh man. That was it.
The country sound. It was expensive, but I really should have bought that one. But… I’m not really a country guy, and like I said it was expensive, so I didn’t. The store closed not too long after and I ended up chasing trying to get that sound again for years, only to finally spend a bunch of money on a ‘58 AVRI Tele that gets close but still isn’t quite the same, and which will depreciate in value over time as opposed to that old Tele, the value of which has skyrocketed. But, I’ve never bought a guitar as an investment — I’m buying instruments to play. Still, these two old Fenders I’ve mentioned have made me think as I am currently in more of a downsizing stage of life and the value of everything is less than what I bought it for, but that Pre-CBS stuff (and even CBS era amps) has gone up over the years.
4) First time I plugged in a P bass

I’d read that a P bass was really the rock bass sound, but I was coming up in more of a Jazz bass era and the best player I knew, a guy who took me under his wing a bit when he saw that I, someone he knew as a guitar player, was very curious about bass, strongly recommended the Jazz. So, I got a Jazz. And it was great to play, but I always felt the sound wasn’t the sound in my head. Finally sent me reading (early-ish internet days) and then shopping. Got up the courage to ask to try a Precision in a shop, plugged it in and I was like, oh man… That’s
it. (for me. I know lots of people have different sounds in their heads, and actually many of my favorite players play Jazzes, including that friend). Still have that same P bass almost 25 years later