I had a recording studio for 10 years and I can assure you that to a recording engineer, soundstage depth is "a thing." Depending on the band and the source material, I worked very hard to create an illusion of soundstage depth using several well known techniques.
Creating the illusion that some instruments and voices are behind others involves using carefully structured reverb and delay on each track besides volume differences and EQ (not to mention how the instrument was actually recorded). The engineer can vary the timing of a delayed reverb, the length of the tail, the frequency response of the reverb, and other cues to make it sound like some instruments are further back than others. Most engineers would be very disappointed to hear someone say that there was no soundstage depth to their recordings.
If you read the recording notes of many modern classical recordings you will see that the engineer uses several microphones placed near the orchestra and various places in the hall. The mixing engineer then can manipulate these individual tracks to create the illusion that you are sitting in that hall. I've got some classical records that have caused people unfamiliar with audiophile sound to sit there gaping in awe. One friend exclaimed loudly, "How does it do that!!!?"
In contrast, some genres like hardcore want zero depth. They want the sound to be in your face, in a flat plane between the speakers. Obviously, this demands a different set of recording and mixing techniques. I can tell you from experience, it's not easy to get that sound.
I'll give you an old example to test. Santana's first album has a very good soundstage with the illusion of depth. If you hear a flat soundstage where all the instruments sound like they are in the same plane my advice is to work on your system. If you haven't been to an audio show I would suggest that you start there. You will definitely hear some systems and recordings that are downright spooky in how they create the illusion of depth.
Here is a relatively quick test to see if you can hear this phenomenon. Move your speakers out into the room and move your listening seat if needed so that you are listening in a very nearfield setup. Put the speakers about 3 or 5 feet in front of you (experiment) and sit exactly between them. You should get an effect approaching that of wearing headphones. The sound should be "holographic." Hopefully you should hear the illusion of depth on good quality recordings. Once you hear this you will "get it." Too often audiophiles constrain themselves with speaker setup that prevents achieving a deep soundstage. For an afternoon, forget WAF or any other constraint and play with your speaker and listening location. Maybe you can't leave everything in the optimal listening position but you you will know what you are shooting for and you can try to get it as close as possible.
For me the illusion of depth is one of the most thrilling aspects of this hobby. Sometimes when I'm listening to a favorite well recorded title I just smile and shake my head. I've heard this thousands of times and it still blows me away.
8th-Note's replay answers the question nicely.
The recording engineer creates how they would like the recording to be heard via the techniques they’ve learned and choose to use. Our equipment and how it is setup can interpret the particulars of what the engineer intended, especially the nuances of depth, soundstage and the imaging results that we’re discussing. Thereafter, we can selectively purchase and set-up our hardware to best present the nuances in the recording.
While specific speaker placement suggestions are generally pertinent. The key word is generally. Because the placement of cabinet speakers tends to generally fit the suggestions.
However, those of us with unique speakers’ types may benefit or not, from such general suggestions. For instance, I do have my ~8' high x 3’ wide SoundLab electrostatic speakers substantially pulled out into the room at approx. 7-feet. But they can be placed within inches of side walls without the deleterious reflection effects that point source speakers have, because SoundLabs are line sources. Also, because their radiating pattern is dipolar, their placement parameters are different than the normal cone and dome speaker system.
