It is not necessary to do anything. Absolute phase is one of those things that is a whole lot more theoretical than actual. Technically there is an absolute phase. In reality every step in the chain- microphone, mixer, tape, etc- can and often times does invert the original phase. Most recordings are a random mix. You can drive yourself crazy trying to decipher absolute phase. Or stay sane. The sane solution is either do nothing (what I do) because it doesn’t matter (it doesn’t) or you can do the next best thing and listen, reverse polarity, and listen again. Only don’t be surprised if you do this and detect no difference. Or you can hear a difference, but then when you play a different recording you get a different result. Which is why the sane thing is to do nothing.
As for the sub you do of course want them in phase with your left and right channels. Except when you don't. It all depends on room and placement. Listen, and let your ears (or a meter) decide.
As for the sub you do of course want them in phase with your left and right channels. Except when you don't. It all depends on room and placement. Listen, and let your ears (or a meter) decide.