One other avenue of processing that could affect, "the sound-stage" would also be phase.
Phase, roughly, is time.
Caveat here, is that to hear phase, you cannot perceive this from one source, you must have two (or more) of something, a reference and a signal or signals. For example two mics on a single source will have a timing relationship, based on their distance from the source(s), and the directionality of the source.
Example 1) Distance: If you are recording a drum kit,
a cardinal sin for the classical aficionados I know, and have two overhead mics one of which is 4ft from the snare, and one of which is 8ft away from the snare. There will be a timing and phase offset in the mics, in how they pickup the snare. The mix engineer, being on the receiving end of this "dilemma," could decide to time align them, giving the snare drum a tighter transient response, while causing more problems for the rest of the kit or they could leave the mics as they are, which could be perceived as giving greater "space" to the snare. Which is "right" would depend on the context.
Example 2) Directionality: If you are recording a snare, and you have two mics that are equidistant to the center of the drum, but one of them is underneath, they will be getting different information. Their distance is the same, however the impulse they are receiving is reversed. When the top mic has the head moving away from the mic, the bottom mic simultaneously has the head moving towards it. Most mix engineers will reverse the polarity on the bottom mic in this case, although not always, it depends on what sounds better.
----
Anytime you have two instances of the same thing there is a phase and timing relationship. Sometimes this relationship is stable, such as bass guitar being recorded both with a DI and miced bass cab. The two takes on this single source will have slightly different timings, and the mix engineer will make a decision about how they want it to sound. The decision they could make is one of those sources being mostly "out of phase" (
shudder shudder ) with the other one, as that could actually be what fits the mix the best. And guess what?! You would never know; as it is a stable timing relationship, and you have no reference.
For what it is worth this phase relationship often corresponds with frequency; so being out of phase at a certain frequency can create a dip there (rather than summation) it might make the instrument fit in frequency wise without even touching the EQ, which is pretty cool!
----
When you have two or more mics in a room that relationship is often not stable, as they are often picking up more than one source.
----
In processing land this can be further manipulated, for example a source can be artificially widened in the left/right domain. If you take a source, such as two mics on a mono guitar cab, hard pan the mics, and delay one side 8-13ms this will "widen" things out. Now is this decision creative and artistic
OR simply mucking about and causing phase problems? That probably depends.
A musical example of this would be
this album, the mix feels spacious on headphones, but can feel artificially wide on many systems, while I cannot state with certainty, I would guess there is time manipulation going on (probably a decision made in the recent release of it for better or worse) or at minimum that the mics were placed quite far apart in the original recording.
Also, interestingly, if you took the same source and doubled it, hard panned them, and did this, the one that arrived first would be perceived as louder and closer, even though the volume is the same. Even a 1-2ms is quite perceivable for a timing relationship between sources.
Another interesting example of phase manipulation, would from Roger Waters album Amused to Death, specifically the track "
Perfect Sense, Part 1," listen to when the piano comes in at 40 seconds, where do you hear it coming from, and where does it move to? On a reasonably good system, in decent room it is jaw-dropping, and while I don't know what wizardry was committed in the mixing process, I would guess it has to do with the phase relationship between the L/R speakers on this particular source.