Speaker Imaging - Do you hear a line, or do you hear an arc??

@prof ,
I wish I could hear those MBL's, they have been on my wishlist for years.

@OP, 
Interesting question.
I will try a critical listening session, now.
Bob

Aside from obvious caveats about the recording, I'd say generally:

One of the ways I know a speaker is really imaging well and "disappearing" as a sound source, is when the soundstaging and imaging can remain deep from the center out to the speakers.  In other words: not an arc with all the depth in the middle.

I think any pair speaker set up in the usual triangle will give you centralized images between the speakers and with some depth.The trick is to get that depth happening at the location of the speakers too.  Intuitively (and not being a speaker designer myself), my sense is that speakers that have some combination of resonances/deviations/colorations can draw sonic attention to the speaker, so that instruments to the sides of the soundstage can feel more "stuck in the speaker" because you are hearing the speaker.  Hence you get that arc with depth in the middle but images pulled forwards to the speaker location to the side.

As I say, when a speaker truly disappears as a sound source, there isn't a sense of instruments stuck in the speakers to the sides, the depth can go way back evenly from side to side.   (My Thiel 3.7 speakers were particularly spectacular at this, my Spendor S3/5s, my Waveform Mach MC monitors, and of course my MBL omnis all do this well).

Depending on how my speakers are set up I either have a shallow arc or one that extends beyond the wall outside.The latter is if the speakers are on the floor instead of stands and I sit on the floor in the sweet spot.I'm guessing it has to do with the large windows being at ear level or not,or bounce from the ceiling even.

Knowing how a full orchestra is conventionally seated on stage, I think we subliminally project that shape, higher and curved at the back, straight across the front, angling out at the sides, on to the cues coming from the recording. Note also, when a sound seems to be coming from farthest away, it’s pretty much always centrally located (center back).

Of course, as is the case with every aspect of stereo reproduction/quality, everything depends on the recording(GIGO). Then too; there’s a wide variety of microphone techniques, not to mention live, studio, and other variables. The following provide tests, with which one may determine whether their system actually images, or reproduces a soundstage, as recorded/intended. ie: On the Chesky sampler/test CD; David explains in detail, his position on the stage and distance from the mics, as he strikes a tambourine(Depth Test). The LEDR test tells what to expect, if your system performs well, before each segment. The Chesky CD contains a number of tests, in addition to the LEDR. Digital download verson: (https://cheskyrecords.bandcamp.com/album/chesky-records-jazz-sampler-audiophile-test-vol-1 ) The CD version, of the Chesky Jazz Sampler, Vol 1: (https://www.amazon.com/Chesky-Records-Sampler-Audiophile-Compact/dp/B000003GF3) An online resource: (https://www.audiocheck.net/audiotests_ledr.php ) BTW; The shape of your ears’ pinnae is also a variable, regarding your ability to perceive images/locate sounds. An old article, from Stereophile, regarding the LEDR test: (https://www.stereophile.com/features/772/index.html ) That article also mentions some other possible impediments, to a system’s imaging abilities. Of course; all that’s only for those interested in finding out, if their systems(and/or ears/brains) are up to the task, given a recording that contains the info. These articles may interest some: (https://www.stereophile.com/asweseeit/1286awsi/index.html) & (https://www.stereophile.com/asweseeit/390awsi/index.html) btw: With my setup, I hear no line or arc. Just whatever’s on the recording. On the good ones; I’m there.