Q: “Which should I use to send vocal tracks to an Aux track to apply effects – pre-fader or post-fader?”
A: Most DAWs let you route an Aux send signal either pre- or post-fader. When an Aux send is configured post-fader, the effect’s wet-to-dry ratio is preserved as you raise and lower channel faders; lowering a channel’s fader simultaneously lowers the Aux send signal level. That keeps you from having to adjust effects return fader levels in the mixer or wet/dry ratios at the effects processor every time you make a level adjustment with a track’s channel fader. (In most cases, you’ll want to keep the balance between a dry track and its effects relatively constant throughout the mixdown process.) As a result, post-fader effect sends are used more often than pre-fader effect sends, although the latter configuration definitely has its uses.
As its name suggests, a pre-fader Aux send is not influenced by channel-fader moves, because the signal is sent to the processor through the Aux send before it gets to the fader. Therefore, the processed signal level from a pre-fader Aux send remains constant, no matter how you move its corresponding channel fader.
One use for this type of configuration is to keep a vocal track’s reverb level constant while you lower its dry level. Try this: set the level of reverb you want the vocal to have in the mix by turning up its pre-fader Aux send, which is routed to your reverb unit, until the effect sounds right.
Then, slowly lower the vocal’s channel fader. As you lower the fader, the vocal’s dry level dips while the level of the processed signal remains constant. The result sounds as though the vocalist is walking away from you: the dry sound gets quieter, leaving just the reverberations of the room.