Most mic-placement advice is vibes. This one is arithmetic, it’s older than everyone reading this, and it still prevents more mud than any plugin: the 3:1 rule.
The rule: when two mics capture the same source area, the second mic should be at least three times the distance of the first mic’s distance to its source. Vocalist’s mic six inches away? The next open mic wants to be eighteen inches or more from that vocalist. That spacing drops the bleed low enough that phase cancellation stops eating your tone.
Why you care: comb filtering — that hollow, phasey, “why does this sound like a cheap speakerphone” effect — comes from the same sound arriving at two mics slightly out of time. You can’t EQ it away, because it’s not a frequency problem. It’s a timing problem wearing a frequency costume.
Where it shows up: choir mics, drum overheads, two acoustic guitarists sharing a stage, panel discussions with a forest of podium mics. Walk the stage, do the math, move the stands. Free fix. Feels like a new PA.
You can’t mix your way out of a placement problem. Move the mic; the plugin was never going to save you.