What makes for a “proper” soundstage for Symphonic music?

Violins are normally divided.

So on an arc, typically you have first violins, second violins, violas, cellos.  Normally, then, violas would be just center right.  Often they're harder to pick out as a discrete group.

I gave general, basic placement.

More specifically,
Front: First violins, second violins, cellos, violas, bass (could be slightly back), cellos and violas can be exchanged OR
First violins, cellos violas second violins, bass.
Then Woodwinds L to R: Piccolo, flutes, oboes, English horn
Next row: bass clarinet, clarinet, bassoon, contra bassoon
Last row before percussion L to R: Fr. horns (L of center), trumpets, trombones, tuba
Harps can be anywhere, but often at extreme R or L.
Percussion can be anywhere behind brass depending upon the shape of the performance/recording space.

Basic orchestra placement is easy hear on any decent recording.
https://alg.manifoldapp.org/api/proxy/ingestion_sources/089584fe-1ad4-4750-923f-6394ffade4ad

The tricky part is fooling our ears to create an imaginary space in our head with the setup. Rock is the other genre that can be difficult to "soundstage". 

At least to my ears, instrument/vocals can be all over the place.

"Live-all members present in the room recordings even during the classic years were becoming replaced with complex technique,(Mood Blues,Floyd,Yes etc.)

Unfortunately, orchestra layout can't apply to R&R.