I've always wondered whether other people see the same orange as I do.
Have you ever noticed you can hear a sound or see an image in your mind, but can't taste or smell
I'm sure sound engineers today have answers, and I suspect all composers today think of the spacing of sounds, put I think too many of them confuse a wall of sound with the wall of music
Tricky question to answer. In one sense, composers aren't all that bothered about 'stereo', and are rather more concerned with the total tonal effect of the music. However, having said that, the relative spatial positioning of instruments in an orchestra is pretty fixed, and sometimes composers will deliberately alter positions to get certain effects. Mahler, for example, had sections of the orchestra offstage in a couple of his symphonies to get specific tonal and spatial effects. Contemporary composers are also keen on altering this. I can recall one (actually pretty awful) modern piece where some of the orchestra were seated in the audience. On a less extreme level, even slight alterations in position can have an effect. E.g. until the early 20th century, the first and second violins were typically on the left and right of the conductor. Now they are all bunched on the left. However, if you listen to some conductors who insisted on the old left-right division (e.g. Klemperer in some of his stereo recordings) then there are at times quite notable differences.
Composers compose with how the orchestra is seated in mind, and sometimes specify where certain instruments should be placed. Many took advantage of how first and second violins were divided and placed on the sides of the orchestra until the 20C.
During the romantic period, as composers became more experimental some began placing instruments all over the concert hall.