What one change to your room, system, or setup made the biggest improvement?

Only problem was that the new room sounded horrible.

What finally changed it for me was purchasing two corner bass traps from ATS Acoustics and placed them on the front wall corners behind the speakers. Best $600-700 I ever spent. There was an immediate and profound difference in what I was hearing. Vocals were clearer and bass was smoother and more even. I was now able to listen for a few hours at a time whereas before, I would get listener fatigue after about 45-60 minutes.

Since the bass traps did wonders, I picked up matching absorption 2'x4' panels for the side walls along with 3D printed diffuser panels I picked up from an Audiogon listing. All of this made an amazing transformation and didn't cost an insane amount of money or require a lot of effort to install. Best part about this is, that it works with any equipment upgrade I may make in the future. Wish I had done this sooner.
 

@bflopez +1: this is almost exactly the same steps and experience I had with ATS corner bass traps and matching absorption/diffusion. Thank you for validating my journey with treatments! 

My Cayin 50 tube amp....I love it....so warm sounding.

Hard to assign true impact differentials, since all these upgrades work cumulatively. Once I settled on my components, I was impressed by the improvements from:
- Room treatment (corner and wall bass traps)
- Fidelium speaker cables and interconnects 
- Townshend speaker podia
- Dedicated power + SR power conditioner 

They all helped with better definition, separation, depth, timing. 

Hard to assign true impact differentials, since all these upgrades work cumulatively.

It might be hard to be exact, but why it would be hard to estimate the difference made by larger changes is unclear. The difference between a room with flabby bass or echoes is a huge change, and the fact that, overall, a series of changes are cumulative does not prevent that observation.

@hilde45 

From my experience, because of 2 (related) factors:

• ordinality & 2nd order effects: some improvements only shine once others are in place. E.g. the impact from the Townshend podia in my room increased notably after I had more room treatment. 
• discrete, step function rather than continuous increases. Once a combination of upgrades (power, cables, ...) jointly lowered the noise floor below a certain threshold, suddenly instrument separation drastically improved. 

Perhaps more experienced audiophiles can factor these interactions into their upgrade path and disambiguate their individual vs cumulative/interactive effect, but for me there were clear moments where the effect of 'the whole' was greater than that of the sum of its parts. I am very much still in the empirical rather than the rationalist phase of system optimization.