Why do my chugs sound muddy?

Short answer

Usually one of five things: your palm mute is too far from the bridge, your pressure changes from hit to hit, there's too much gain, your low string is too slack for your tuning, or strings you aren't playing are ringing. Turn the gain down and record one bar to find out which.

How to find the problem

Record one bar of muted 8ths with the gain lower than usual, then listen back.