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

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).

@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

The dog barking on Stereophile test disc 1 sounds well outside and to the rear of my speakers (diagonally back and away, pretty much into the front corners of the room).  I suppose that counts...

Dodecahedron.

When listening to a system you feel images well, how do you perceive the sound stage?

With my ears.

Do you perceive it as a rectangular space on which the speakers sit, or does it sound like an arc, going further back towards the middle?

With which recording?