I'm getting into mixing the last month as it's a vital role in publishing my music for sale online so here goes my question.
When would you use mono reverb on an instrument and for what purpose/s?
When would you use stereo reverb on an instrument and for what purpose/s?
"Stereo reverb" may mean different things:
The latter category is further split between "true stereo" and "dual mono" categories, and maybe something in between. In dual mono the left and right inputs have completely independent and separate processing, and sound doesn't "leak" between the extremes, which is a very unnatural situation. In a real room you can't prevent sound from echoing from wall to wall, even if you place the sound source to the furthest location on either side. In "true stereo", a stereo signal is kept as an actual pair and placed in the device's internal stereo field or virtual room or whatever, as a pair. Not left and right separately to their own independent mono reverberators. How "true" it really is, is somewhat debatable, because it's an artificial effect anyway.
The send effect buses in some DAW applications and mixing boards are mono, so if you use such a send for reverb, your signal is first summed to mono before being sent to the reverb. Or the reverb effect itself may sum its two inputs to mono before processing. That means that even if there was actual stereophonic information in the source signal, for example if it was recorded with a stereo mic, it is narrowed down to mono before the reverb gets it. This may or may not be OK, depending on if you like the sound and if the signal is "mono compatible", i.e. can be summed to mono without causing too much phase canceling. You should have your signals reasonably mono compatible anyway. What is too much, is up to your discretion of course. If you want to have a "true stereo" reverb, use it as an insert effect in a stereo track, if you can't set up a stereo bus.
Even if an effect is stereo in, stereo out, it may not be "true stereo" in everyone's opinion, if it's not properly placing the left and right inputs separately in its internal stereo field, keeping the input's own stereo image intact. The reverb may even do an internal mono sum to its inputs, or a phase swap for one channel, or anything. It may or may not be suitable for the type of sound you want. Search for "true stereo" if you're interested.
Use stereo reverb whenever you think it's good for the genre and the sound. Or simply if you like the way it sounds. A really wide stereo field sounds nice to me, but YMMV.
It's worth noting that (mono-in stereo-out) stereo reverb is one way to artificially create a stereo field from a mono input. Other stereoization methods include comb filtering with a delay on one of the stereo channels, complementary comb-shaped EQ curves for left/right, and special stereo chorus effects.
Use mono, as in "mono out", if you like its sound for the genre. For example if you're after a specific retro/lo-fi sound like a reverb through a guitar amp or lo-fi reggae sort of sound. The method of operation of the reverb is also important, and might be more important than if it's mono or stereo. For example, spring or plate reverbs have their own sound. And a spring reverb cannot really even be "stereo" because of how it's constructed, maybe dual mono, but not real stereo.
I think you'll find out by listening if you like your reverb in stereo or mono. Either way it's your own sound you're creating.
You can spoil your mix and make it muddy and unclear with too much reverb, and it doesn't matter if it's a mono or stereo reverb. Both kinds of reverb can be used wrong.
You can spoil your mix with bad stereoization by making it mono-incompatible, or just smearing the wrong frequencies in a bad way. It doesn't matter if the bad stereo field comes from stereo reverb or other methods.
It's not clear to me whether or not you understand the difference between the two but since you're asking "when" and "for what purpose" here are a couple of use-cases:
stereo: you have a mono sound source (let's say a tambourine recorded with one mono mic) panned to one side. You could send that to a stereo room reverb (post-fader but pre-pan) to create the illusion that it was recorded in the same room as other instruments -- less isolated, more "live" etc. Or post-pan for a different take on the same concept -- the illusion of space.
mono: you have a mono sound source (let's say a mono synth melody) panned to one side. You could send it to a plate reverb (post-fader, post-pan) for a very directional "effect-y" sound, which would tend to emphasize the pan. If you change the pan position to the opposite side during the mix, the reverb will travel with it.
There are of course any number of ways to creatively use reverb so this is strictly "for example," since you asked.
