If there is one thing you could change in your current system. What would it be?

I will change nothing right now because there is synergy between all the pieces of my small speakers system and i optimized everything (mechanical,electrical acoustical) to reach their peak working level together...(it takes many months to do that)..

I will change nothing now because the pieces cost : dac,amplifier,pre-amp tube equaliser, headphone amp, powered speakers, linear power supply,conditioner  are all at the same low price each pieces under 200 bucks each most around 150...

This is my secondary system for casual listening and non focussed listening...

I will change nothing in my main system: AKG K340, Sansui alpha, and Hidizs dac + file bank+equalizer... because this system is a TOP one already at peanuts cost (i tried to upgrade the amplifier and i failed the Sansui alpha is a TOP amp even for today and the K340 is very hard on most headphone amplifier...

But if budget is not a problem...

It is very easy to improve my secondary system with new  TOP speakers (driven now my my Sansui alpha instead of smaller powered one so good they are right now as modified and optimized, i will replace them with 1,000 bucks non amplified speakers probably...

i dont think i can replace the AKG K340 optimized with any headphone under 1,000 bucks and by most costlier headphones for reasons i will not repeat here...

My main improvement anyway in the last months was buying a better  non linear supply, a better purifier/conditioner at price lower even than the price of the pieces they replaced...

But my greatest impactful change was not buying gear ...

it was the specific grounding of ALL pieces of gear in my 2 system...(even the linear power supply, the computer, the conditioner were grounded separetely with all the other gear piece separately)

Epochal improvement...

As powerful as acoustics controls if i want to compared the change...Lower noise floor  impacting my 2 systems...

THe K340 is now a king among headphones...nothing i ever own even touch it...

Nothing... or everything. If I suddenly could afford to double my investment... then I would start research. 

Agree with @immatthewj in adding transducers of types. Maybe even immersive audio. 

Small Core Audio Designs ceiling cloud. Arnold the proprietor has a fantastic music room and imo product.

Nothing can improve my system sound. Everything (source, amp, cables, speakers) in my audio system are the best and most transparent sound in the world. Only way to improve my system’s sound will be modify (by me) the expensive (>$10k) SACD player (or streamer)

In 2nd thought, I don’t think that will improve my WT-95 sound because the new chip technology destroys the original musical data and the sound won’t be better. I've modded ESS 9038pro DAC and Chord Hugo2. Anyway, separate DACs can’t sound better than All-in-one WT-95. Every separate DACs always sound veiled and distorted. As long as audio companies don’t know what causes sound distortion, the separate DAC will always sound veiled and distorted. I know why separate DACs sound distorted but there is no way DACs to avoid it.

I won’t mod any $10k SACD player. The sound improvement will be noticeable but it won’t be worth extra $10k. Also, my mod is hard and needs many special replacement parts which are are hard to get for $10k players. Alex/Wavetouch audio