Guidance

The guidance I give has been provided in many similar posts - that making incremental improvements or optimizing current systems needs to start with listening to acoustic and amplified music in intimate venues and establishing your perception on how recorded music should sound on your system.  Then you should choose reference recordings you feel approach your perception on live music best.  The next step is to listen to your current system using those recordings and establish a goal for improvement.  Then, when you audition equipment, ancillaries, and tweaks, you can determine if you have met your goal.  
You seem to have established a goal to improve the soundstage and improve the weighty-ness (image density) coming from my current system without a large spend.  I will give suggestions from my experience:

1. Optimizing speaker placement.  I have found small adjustments toe in, separation, and distance to listening position can snap in staging and density. 
2. Room treatments. 
3. Power conditioning or regeneration:   A good power product will improve many areas of SQ including staging and imaging by reducing noise floor. 

However, to get a transformative change, and reviewing your system, a capital expenditure to improve the source components (streamer and DAC) and the amplification (pre and power, or integrated amp) is required.  I would add a Burmester 082 or 032, AR, or Hegel to your list of integrateds.  All are known for image density and staging.  The Wyred4Sound 10th Anniversary, Bricasti M1 or M3, AR, and Mola Mola Tambaqui are also known for staging and imaging.  

+1 for adding a pair of REL subs. The high level connection which attaches directly to your speaker terminals on your amplifier, allows the subs to blend seamlessly with your speakers. I recently added a pair and I consider it to be the best bang for the buck upgrade I have made.

Adding a sub makes a huge difference!! If you tune it properly, it will totally disappear to the point you will ask yourself if it's turned on. It's something that I wish I did sooner on my system. It allowed me to move my speakers where they mid/high sound best, without loosing bass. With my tone controls set to 0 now, the amp isn't trying so hard to produce bass, so everything just sounds cleaner, fuller. 

You may also be siting in a null point, this will kill the sound. Play a song you know well, then walk around the room, does the bass get bigger/smaller as you walk around? How does it compare to your listening spot? Also make sure your speakers are in phase. Again a song with bass you know, turn the balance full left, listen, then full right, listen. Is it louder or softer at the extreme L/R? If so, they are out of phase. 

Of course moving the speakers around the room, can make a massive improvement! Also adding spikes to the speakers can enhance the bass.

I'd invest in an 'adequate' sub of your taste/budget just to 'dip a toe' and see what happens.  If it seems to moderate what you hear as a deficenceny, stay put and attend to the room....  If it doesn't have a level and xover controls, walk away; used ought to be fine....
Attend to the room....I've never really had the opportunity to delve into that other than simple cheap tricks, so there might be a chance at the big leap forward.

There's your homework... ;)  C'mon back when you've muddled about with the dragon, as there's really no strangers here....

Well, mostly...*L*

When you mentioned DarTZeel and said you didn’t want to break the bank, I almost inhaled my gum!  Could buy a house or buy DarTZeel!