Is soundstage DEPTH a myth?

It is no myth, and you don’t need to spend a ton of money.

But you do need to get things right.

The speaker spacing, both to each other as well as the back and side walls, is very important. You also want "time alignment" between the drivers.  And you want symmetry in the room. Soundstage/imaging/depth are all dependant on constructive as well as destructive interference and phasing and for these to occur properly, both speakers essentially need to be the same and "see" similar environments so they are the same in the seating area.

And..., if you want to feel "encompassed," you’ve got to bring the volume up to realistic listening levels!

I know it sounds cliche, but I’ve spent years putting together and perfecting a pair of speakers that image like no others I’ve heard. And I have JBL L200/300, JBL L112s, Altec, Big Red Supers (triamped), and Chartwell LS3/5As, and have heard Magico and Focal at demos, as well as lots of other varieties at listening parties.

If you get down to the Orange County, CA area, you are welcome to come by and hear what it is all about. Meanwhile, I leave you with this YouTube video. This is just the two inner speakers playing with no sub attached, no eq, and no room correction, either physical or electronic. The room is large (27’ x 16.5’ x 7-1/2->15’ ceiling, and open to the entrance hall and dining room.

This is an SACD played on an Oppo95 ($300 used) through a Yamaha RX-Z9 RECEIVER ($4,050 new at 10% off in 2002) in "Pure Direct" mode. The speakers use a JBL 18" 2241H, JBL 2251J, and ESS Great Heil that I’ve modified ($1,500 total investment with used drivers but new crossover components from ebay).

This shows the spectrum in the seating area without the 18" connected to alleviate the "noise" associated with floor and room bounce. I know it look a bit "bright" but that is a personal preference and I think it makes music more lively and brings out the microdetail. As shown, the crossovers are all the way up and there is even a "flat switch" that removes the upward tilt, engaged for overly bright or noisy cuts.

All caps are Audyn Q4s, all resistors are Dale 1%, all inductors are heavy-gauge air core.

So what do they sound like? Well if you believe in YouTube (many don’t) put on some really good headphones and take a listen. And while you’ll hear a nice smooth response, and lots of detail, you loose the imaging, but are welcome to come by for a demo any time. This was recorded on a Nikon D750 DSLR.

https://youtu.be/oLgQCHmXSUU

The Behringer mic used to display the spectrum is mono. You cannot "analyze" mono pink noise through both speakers simultaneously because of constructive and destructive interference in the room. I've shown only the left speaker, but the right is similar.  The audio recording was made with a Nikon D750 DSLR camera.

Yes the recording is more reverbrant because it picks up the untreated, big, room acoustics. The brightness is my preference (but also the room) and could be turned down at the L-pads or flattened to a more conventional curve with the switch on the crossover.

Buil in mic..., so of course separation suffers. But the Nikon actually does a pretty good job, and lots better than many cell phone recordings I’ve heard.

"About soundstage, I've owned TOTL headphone and I still couldn't notice soundstage. I've clearly sense what's in front of me from front left to front right. I've also heard things up close to my side left and side right. This kinda qualifies as soundstage depth. But when it comes to the front depth, that I haven't clearly heard."

If we agree that soundstage and imaging are at least partially the result of the engineer using constructive and destructive interference to create "peaks and nulls" across the room to create the illusion of a soundstage, wouldn't this be expected of headphones where there can be no interference between the channels let alone some predetermined distance between the speakers that the engineer was listening at?

"Are you saying depth is a result of sound bouncing off the physical room? And since there’s nothing to bounce with in a headphone, there is no depth? I’m sorry for not understanding."

Not so much bouncing off of stuff in the room, but the interaction of the sound waves of the two speakers playing simultaneously in the room.

Something to consider. If both speakers play the same thing, in phase, there are constructive as well as destructive interferences (peaks and nulls) set up in the room. For something to be dead center, both speakers are playing the same thing, at teh same volume, in phase, reinforcing each other and sound is therefore louder at that spot. Similarly, depending on the wavelength and distance between the speakers, you could have a path differential that creates a null when both speakers are playing the same thing

But on headphones, the center is in your head and the sound of both channels is not reinforcing a wavefront making it louder, or quietier, if you are listening to a null spot.