Creating a Large Sound Stage

Everything matters, but some speakers just image and throw off a big 3D soundstage better than others so it greatly starts with that, and placing speakers too close to walls squashes subtle things like reverb trails that contribute greatly to soundstage size and sense of space.  So everything matters, but starting with speakers that have these qualities and positioning them properly is a huge part of it, and of course room/treatments also come into play. Just my $0.02 FWIW. 

It is necessary to control the room acoustically...

Any relatively good pieces of gear can be used.

The main factor is understanding of acoustic concepts and how to use them and modifying them...

Reflections/diffusion/absorption devices in balance and well located...

We can use DSP or mechanical devices to modify  the  speakers behaviour in the room in relation to a listener location...

We must take control of the pressure zones of the room ( i myself used Helmholtz resonators grid but it is unesthetical and impractical for most people then you must use DSP ) 

The most important concepts are : Timbre with Reflections/diffusion/absorption ratio and ASW/LV ratio ...If you dont understand what they are you will buy gear upgrades till your death without never creating an immersive soundfield... 

The gear pieces must be synergetical, and relatively good pieces (at any price); the focus on the gear impact made by most  hide the essential : acoustics concepts and parameters  controls including psycho-acoustics...

My low cost system in my first dedicated room gave me a total immersive soundfield completely out of the speakers plane, including my listening position, with in some recording, voices speaking to one of my ears and an orchestra playing behind the speakers at the same time ...Thanks to acoustics basic understanding...(It could have been improved only by Choueiri filters  but i was happy as it was) 

This experience had nothing to do with my gear design value or power which was good but low cost: vintage speakers (mission Cyrus) and amplifier (Sansui Au 7700), low cost dac (SPS nos) 

The room, how the speakers are placed in a room, and the speaker’s dispersion character. 

Based on my experience of conducting level-matched comparisons, upstream components have little impact on soundstage width. The exception is tubes vs solid state, where the former can create a slightly more expansive soundstage, probably due to the harmonics. 

Nah, it's because the electrons are free to travel without hindrance from all that substrate.

probably due to the harmonics

@soix

+1

Definitely, everything matters. However, some things are of top importance. First the speakers, the placement and the room. Then acoustics and the electronics. 

But it still depends. Like you want incredible holographic sound out into the side walls. and deep past the .. Bermeister amp and preamp with top of the line dCS or similar electronics... or high tier Wilson / Rowlan with great speakers... Sonus Faber, Wilson, or Magico. 

I have heard many systems like this... with your eyes closed you would never have a clue as to where the sound was coming from.

My system does this... it is warm and natural sounding... versus the sonic spectacular you get with my examples above. Above you get ultra detailed and somewhat unreal and spectacular... incredible three dimensional space. I always go away  in awe, but would  not want it in my audio room. So, you must be careful on the flavor you choose.