A deeper more holographic soundstage.

besides room treatment the next important step is to get your digital equipment on vibration control gear. One example, my transport sits on TR bases from marigo, then his special shelf, marigo 4 40 millimeter vibration spacers, a granit mdf shelf, 4 six in. springs under that, and then another granit mdf shelf and then the final footers some kind of ball bering mini devices under that. Not only are taking out vibrations from your equipment but you are also dealing with earth vibration.I did this over ten years ago. More analog, three dimensional, cleaner sound than I ever thought possible. Vibration vibration vibration. This is the key to better sound.

Marigo audio is one of the kings of vibration control. Give him a call, he will help you greatly.

Look into BACCH4Mac or HAF digital signal processing.
I have no experience with HAF, but am using the BACCH4Mac Intro edition and probably will upgrade to the Audiophile edition (although it is rather expensive.) BACCH is a product of an acoustic research lab at Princeton University.

BACCH minimizes/eliminates crosstalk, manipulates the signal in the time domain, and can create rather dramatic effects on soundstage/imaging.  I'm told that effects from the more sophisticated ($) versions, "Audiophile" and up, can be breathtaking.  My own experience is that more magic occurs on some recordings than others. 
You won't hear much difference on, say, a Diana Krall recording (where all the attention is on one center image). But on one binaural recording, the apparent position of a trombone moved from at/near my left speaker to about 2 meters to the left of the same speaker.  Not only the speakers seem to disappear, but the walls of the house can seem to disappear.  It can increase the apparent depth and reverb. It does all this allegedly with no impact on the tonality (it is not the same in effect as room correction software or an old school graphic equalizer).

I have highly directive, hybrid electrostatic speakers.  These seem to work better with BACCH than some other speakers, so YMMV.  There is a 2 week trial of the Intro edition (and it's just software, so if you are not happy with it you don't have to ship anything back).

So, while room treatment may help and equipment rack placement may help, you can get to this level of imaging without it.

Interesting... I believe you because all room are different and dont need the same level of treatment, and some Audio system are already synergetically and by itself already very high S.Q. then i belive you...

BUT it is not true for most of people at all....In most case acoustical treatment and controls are necessary....Beside that except for lucky buyers, most very good system cost high price...

My speakers paid 50 bucks are very good but not on the level of your ATC for example...

But guess what, my S.Q. is probably near you, i listen to a soundstage holographic with perfect imaging, natural timbre with audible decay,bass i feel in my stomach, and the sound is filling my entire room at low volume ...Then all situation are different...Thanks to my homemade embeddings device... Without that compared to your ATC probably my speakers are way under in S.Q. term...

We cannot deduce that all costly system will be good without the need of any embeddings controls at all and will not need specific acoustical controls nor electrical or mechanical controls either...The general rule for most small room and audio system is they probably badly need all treatment and all possible controls...

By the way  i apologize to say that downright but you probably dont know what your own system is able to do more with a complete implementation of mechanical,electrical and acoustical controls...Perhaps your system even if you are satisfied is not working, unbeknownst to you, at his better level possible...

Most people unsatisfied or not, are not conscious at all of what are the really true high potential level of their audio system, what it is able to give when working in optimal controlled conditions... This is the ONLY experience and certainty i have acquire at the end of my journey...I called that embeddings controls in the 3 dimension of audio....

Happy New Year...

I've heard two pieces of equipment that increase the depth of the soundstage - one I own and the other I heard at AXPONA.

A company called BSG Technologies made a processor called the QOL (pronounced coal). This unit is a refinement on phasing processors such as the Carver's Sonic Hologram Generator, which I also own. The QOL is more subtle than other processors of its type but its main benefit is that it deepens the soundstage. I use it over 90% of the time when I listen to music. It was well reviewed by TAS and Stereophile so you can find more information if you Google "BSG QOL review." Originally they cost $4k but you can occasionally find one on the used market for under a grand. I like mine so much I bought a spare from the guy who ran the company. 

The other one is the Synergistics Research Atmosphere Infinity. I heard this at the 2018 AXPONA where the company owner put on a really good demonstration. I didn't hear much difference when he added/removed the little HFT thingies but the soundstage absolutely collapsed when he shut off the Atmosphere Infinity unit. He had the system playing with all his tweaks before the demonstration and I remember thinking that this was the best sound at the show when I walked into the room. It really sounded like musicians were playing in the room. This skinny tower goes for about $3500 and if it does at home what I heard at AXPONA it's well worth it. I'm looking forward to auditioning one at a dealer when I get a chance..

Just for the record, I have a fairly optimum setup with a large (16 x 35) room. I'm running Thiel CS6 speakers which are several feet away from the front wall and carefully set up. My system sounds really good without any processing but the QOL adds the cream of increased depth. On a good recording I've had people sit there and gape in disbelief. It's very three dimensional.