Biggest "bang for the buck" audio gear you have owned(in your history)?

The biggest improvement I made, and not for killer dollars, was when I bought my first pair of serious speakers--Magneplanar MG -I.

1. Vintage tubes. For around $100 each I’m not sure there is a better bang for the buck out there.

2. Empirical Audio Off-Ramp 5.  This USB converter absolutely transformed my digital system for less than $1k used.

3. Audience aR2p power conditioner for $695 new. I’ve since moved on to the TSOX version at considerably more money but the incremental improvement really didn’t justify the cost. The entry level version is a tremendous value.

Thorens TD-160 paired with a Stanton 681EEE cartridge. I paid around $300 total in the mid-70s, best money ever spent for ROI.

After a number of years fiddling with audio I settled on an AR turntable with a Shure v-15 type iii, a Dynaco PAS 3x (bypassed tone controls), a kit-built ST-70 with Mullards, and FMI stand mount speakers (using their own heavy gauge wire). That modestly priced system woke me up!  I had by far the best sounding system in the whole dorm!
Forty years later, I'm blown away daily by a pair of Harbeth P3esr speakers driven by Odyssey electronics and a Hana SL mounted on a Technics SL-1200GR. For my listening habits and room this system is amazing.
Some may blush at the thought of a $10k system being as good as this. I don't obsess about my system at all and just listen to the music.

I downsized my $100k+ all Shindo rig to a pair of Kef LS50 Wireless II with matching KC-62 Sub. I'm not about to say it's better, (it's not) But running Roon directly into them it does blow the doors off most other systems I've heard at similar to double the price. None of which is important, what is important is that I now listen to it daily vs. having to find time to fire up and sit down to listen to the "big rig". Now I can sit and listen, have it in the background, crank it so I can hear it out on the deck, impress the neighbors etc...