1. Tannoy speakers with Dual Concentric drivers.
Early 2000s I was transitioning from headphones to dipping my toes into speaker waters, and most of the speakers I'd heard to that point had very poor "coherence" (for lack of a better term), where I found it hard to enjoy music. When I first heard these, it was like everything snapped into focus. I wanted to throw my new Polk LSi15 in the trash lol. I still run Tannoys today. Probably always will. I like both tulip and pepperpot driver variants - each has its pros and cons!
2. Koetsu cartridges.
Circa 2007, I first noticed these in the MusicDirect catalog at insane prices. They were pretty little boxes, but the prices were too ludicrous to be true. No WAY that price is justified, I thought. Then I got a chance to borrow a crusty old Onyx (vintage 80s/90s model) - why not? It completely blew away my new Ortofon Windfeld MC. I was gobsmacked. Pure, unfiltered music magic. It was like the Tannoy speaker thing all over again - like I was hearing all my music again for the first time. I was hooked. Now my little Koetsu collection is like family, and will not be sold. RIP Fumihiko Sugano.
3. Stax Headphones w/ T2 amplifier
After various sessions with SRM-007 left me unimpressed, I first heard SRM-009 with a DIY T2 amp (quite a beast) circa 2014, and it was quite clearly the pinnacle of headphone achievement. It still is IMO, though others have closed the gap and now compete as alternative flavors. The DIY T2 amp absolutely clobbers the (to be sure, excellent) Blue Hawaii SE in direct comparison.
4. VAC Master 300 Monoblocks
I didn't expect these to be a significant upgrade over my VAC 200iQ monos. My speakers are 96dB efficient! But I agreed to give them a listen at home - because that would cost me nothing, right (lol)? They were a freaking HUGE upgrade. Several months later and I STILL can't believe what I'm hearing. Absolutely transformative effects. Single most significant component upgrade in my history. The only amp I've heard so far that does EVERYTHING right and NOTHING wrong (other than price haha)