The illusion of soundstage.

I imagine you could think of depth itself as a direct result of width in 2-channel playback. 2 speakers are already casting, according to their dispersion patterns, a given width into the room...where those patterns tend to overlap the most is where you get the most depth...yes?? So, to me, width is sort of the precursor, if you will, to depth...it's all the influencing factors of room, gear, etc that dictate how much or how little of depth or width in a given setup we can attain.

But if you are looking for some inherent cause, I would start with the dispersion capabilities of the speakers (perhaps especially the drivers) and, as well, the soundstaging of the associated gear and/or wiring. And it may take an unbroken string of an alignment of these factors to achieve a particular desired result - from source to room.

I've had speakers that, with the particular DAC I used to have, could image a full 5 and a half feet outside the cabinets, albeit at the expense of some depth, most especially at the rear corners. Now, I kinda have the opposite...only a foot or so outside, but with uncommonly good depth all around.

In my quest it has also been a give th and taketh away scenario. In looking for more depth I lost imaging width and settled for a middle ground. This was done moving speakers in and back and wider and narrower. So room and speaker interaction. The rest of chain certainly contributes but less. Except in my exp. tubes do seem to synergisticly encourage a bloomy boundarylessness that I like. 

Ivan,  5 and a half feet! How far out are sidewalls?

Sidewalls were (and still are) about 6 feet on one side and the 1st reflection point on the other side doesn’t exist since there’s only 3 feet of the side wall extending out from the front wall before it gives way to being open to the rest of the house. I have only owned monopole speakers, but I really noticed that a portion of that uber-wide effect wound up being curtailed just a bit once I switched from that DAC to a (overall better) CDP. That DAC was good at scrunching up the soundstage toward the zone in front of the speakers...something I was not really shooting for, despite its own advantages. (If you’re into the very forward presentation thing, that DAC probly would’ve sounded great with a Grover Huffman loom!...DAC is an EVS-modified Behringer DEQ2496).

If you put your drivers out of phase with each other you will get sound all over the place. Q-Sound is software that manipulates phase to put sound beyond your speakers , even behind you
Alan

Or, in my case and situation, I'm running Walsh drivers which are omnis.  I've a 'soundstage' that 'extends' not only to the sides but also in front and behind the drivers.  Placement within the given space and the surrounding objects (or lack of same) will certainly effect 'the effect'.  I've also noted that from selection to selection, artist to artist, album to album there's distinct differences in what is perceived where.
Adding a second pair, either in 2 channel or 4 channel arrangements, and playing with delay or no, you can be sitting 'on stage' or row 5....
But 'immersion' isn't to everyone's taste.  To each...