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

@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.  

Speakers equipped with top quality drivers. 

Pulling speakers way out into the room. The entire space between speakers and rear wall became the soundstage. 

The first was adding room treatments, both absorption and diffusion and then it was going from 2 to 4 subs. 

The 1 thing that had the largest impact was using and learning about acoustics measurements as it pertains to ER (early reflections), DT (decay time), and FR (frequency response).  The OmniMic software tool allowed me to (mostly) “see what I’m hearing” which started me down the room treatments path.

I’d say curiosity was equally important as the measurement software as idea after idea flowed and testing followed which resulted for me in a long and large learning curve.  Now I help others analyze and decipher their room measurements and recommend treatments accordingly.