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

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?

Soundstaging should not be a line or an arc!

It should be ‘beyond the walls’ and with three dimensional individual images.
Not flat ‘paper cut outs’ staggered in space.

Best enjoyed in a dimly lit room, in the right psychological mood, with sonic clarity of good equipment.

It may be realistic or totally fantasy, depending on type of music.

The best speakers for big, realistic soundstage, are big panels (Magnepan or electrostatic), that focus the sound to a "sweet spot" like a lens.