I have read that it is a mistake to mix by addition rather than subtraction. When I find that certain sounds are too quiet in my mix I will usually lower the levels of other sounds I know are likely drowning these out. Is this good practice?
Since most mix engineers end up with too much level as they push individual instruments up, subtractive mixing is a good technique to learn. But technically speaking there’s nothing wrong with mixing by either addition or subtraction. But if you regularly get into trouble by losing headroom in your mix bus and finding yourself constantly pushing individual faders up and the master down, then you should become proficient at subtractive mixing. But it’s not terrible if you are suffering from fader creep to periodically say, “all right, I’m right back where I started except the level is higher and I’m overloading, so I’ll drop all the individual faders the same amount, say, 10 dB, and restore the master to 0 dB and start all over again.” But as your mixing techniques mature you’ll find that happening less and less, cheating things down if you see the measured level getting too close to full scale.