What was your most fun and rewarding upgrade?


I've got a decent setup:
PrimaLuna Evo 400 Integrated with KT120'S and Psvane 12au7s
Clearaudio Concept Wood with Hana EH Cartridge
Musical Surroundings Nova III with linear Power Supply
Ps Audio P10
Focal Aria 936
No upgraded power cables, but Big Sur Interconnects and $200 Wireworld speaker cables

What I'm curious about was what was your most rewarding upgrade? Also, if you were in my scenario, where do you think the most fun upgrade would be for you? What was your most wasteful or least interesting upgrade? What steps do you wish you would have made first and which would you have saved for last? Also, what upgrade did you love that maybe didn't give you the most bang for buck or was least valuable, but you enjoyed the most?

I'm considering a Herron Phonostage for my next experience. Or cables. Or isolation. Ahhhh the options (but the Herron phonostages...)
128x128j-wall
navships cables/Mundorf/Takman/Sparkoslab/Telefunken/Svetlana/F&T's all made  very happy high fidelity gains. 
Biggest bang for the buck, was room treatment.

Second was the electrical grid, maintain 120VAC, protect against surges (quickly) and high quality cabling and outlets.

The best improvement overall, was going to BI and Tri amping, with individual bass box control, The use of MB columns in a stereo setup, with my current monitors along with the control I have per enclosure and sections within that enclosure, have REALLY been the ULTIMATE TWEEK!!!

Things like, Sparco opAmps and regulators, valve rolling, and cables do fine tune them a step further, though.

Regards
The most rewarding was my last : A used SOTA Sapphire III TT, a new Jelco 850 MK II arm, and a Soundsmith Zephyr III cart. No doubt, the most satisfying upgrade, to date.

Prior or that, my Vandersteen 2CE Sigs. It will take a serious upgrade to replace them. Now, it’s my ‘bang for the buck’ winner.

Wasn’t wasteful, as I sold it for what I bought it for, but an original Adcom Pass designed 535 amp. Always wanted one back in the 80’s, but after getting it, didn’t ‘do a lot for me’ after a while. Replaced it with a B&K 125.2 amp, which for me, was a huge improvement sonically. That’s now gone, and replaced with an old Belles 400A, which I like a lot, and a worthy successor.
Moving from living room to basement as a dedicated listening room.  I can put speakers wherever they sound best, it’s isolated from rest of house, and cement floors much better for sound — especially bass — than suspended wood floors. 
Biggest effect for me was replacing a Magnavox console with a Fisher three piece system.  -- My first step into the exotic world of audio components. lol
Measuring impact per dollar:
Springs under (heavy) turntable.
Replace rack with concrete  blocks
$10 interconnects that bested a $275 set.
Replaced turntable bearing
Replaced stock tubes
PTFE tape on glass fuses


It would have to be my most recent upgrade to my new Bryston 4B3 amplifier. A new friend came over today and happened to hear music playing and asked to see my system. I had no idea he was a musician and had a few albums on Qobuz. So I put him in the chair and played his own album and a few others. He freaked. So he gets home and wants the list of every component after asking a bunch of questions. His Uncle is an Audiophile and wanted to know more as well. His quote from his text " I can’t handle my stereo now, So Bad, And its better than 98% of households. Yours is like other worldly bro. My Uncle want’s to sell his car now and go shopping. 12k? That’s encouraging, tho that’s a fortune. I thought it was a 30k setup based on the sound. I can’t imagine it better than that. "
Yes, Rewarding to hear confirmation from someone else who spends a lot of time in a studio and knows music. He had never experienced Maggies properly driven in a treated room. I have no other friends interested in audio so this was a first. He heard stuff on his own album he had not heard before, including his breathing. (Acoustic with no vocals). Needless to say I am pleased because I have never been able to get an educated opinion and to me it is normal everyday music.
My Mytek Brooklyn Bridge. A universe of music at my fingertips. In the best fidelity I've encountered in my sixty-plus years of listening, whether the source is vinyl, CD or the internet. Could I do better than the BB? Probably. But at the moment I just don't care.
My current speakers I got about 2.5 years ago. I'd really struggled finding speakers I loved flipped through a bunch then we bought our small condo I traded for a pair of original JMlabs Micro-Utopias. I liked and ran them for almost 10 years,  then I went to my first audio show. I realized something was missing,  a more dense fleshed out midrange.

I traded those speakers, and cash, for my current Audio Note J-lx's and could not be happier, This is the sound I've always wanted the 8" driver really gives me a great midrange and plenty of bass for my small room. By far my best upgrade and I've had some good ones in the past.
@phcollie what's your friends name and style of music? I want to look him up on Qobuz.
@jond 
Send me a pm via marketplace feedback and I will discuss. I did not get his permission and would never out somebody on a BB.  Acoustic guitar is his style solo but his taste is based in roots & Indie.
I agree room treatment. Last week I built my own frames and used the knauf insulation from GIK. 6 of those and they removed the boominess and edge off the sound. Music is more relaxed to the ears and listenable for extended periods of time. 
I agree room treatment can be very helpful depending on room and speaker placement options but for me my speakers have plenty of room to back and side walls and i listen semi nearfield- acoustic panels and traps helped but not a big bang for the $. 
My personal biggest improvement for the $ is interconnect cables.  
Finding the right ones that complement the rest of the system elevates the sound to a level that is not possible without them.  
I'm not talking stratosphere either- just $300 per set with discount (Nordost Blue Heaven) but a small percentage of overall system cost.   
The sound became richer, more dynamic and detailed.  They took a few days to settle in. 
Your system simply cannot live up to its potential with basic interconnects.