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

OOPS,  Just saw the last sentence:  B&W 803's with 10" subs underneath, time aligned placement, slightly sunken room open to main room, with identical amps driving all four speakers with separate, identical preamps on, shall we choose, Rosie O'Grady's Direct to Disc (Aphex enhanced of all weird things), "Good Times Jazz Band", a late 1970's recording by the large house band at Rosie O'Grady's Good Time Emporium in Orlando.  This recording has everything from a triangle intro on "Japanese Sandman" to a romping tuba extended solo on "When You're Smiling". (All Audire electronics including head amp, 125/250/400 (8,4,2 ohms) wpc x 4, with an often used Hafler amp and preamp, for comparison.)

I definitely experience a 3D arc soundstage from a lot of recordings, with some elements of the recordings projecting as far as 15ft behind the speakers. Something I have noticed with a lot of recordings is that the vocals are placed further back than the instruments. It sounds impressive but not always as I think it should sound. This could simply be a common ‘style’ of recording, but I think it might be less apparent if my listening position was further away from the speakers. Unfortunately, the constraints of my room means my listening position can only be 7ft away from the speakers. 

with a lot of recordings … the vocals are placed further back than the instruments

On pop records this is probably incorrect except for solo guitar, sax, etc.
On live opera, correct.