To summarise what we've gone through above, the effect of your master bus compressor should be barely audible and provide little to no coloration. It is the lens through which your project will be mixed and should only serve to tame transients and glue the overall project together. Here are some suggested settings to get started with, which can be tweaked to taste: Ratio: 1.25:1 - 2:1. Take a 2:1 ratio - for every 2dB, 4dB, and 8dB that passes the threshold, the compressor will only allow 1dB, 2db and 4dB respectively. Anything beyond a 2:1 ratio therefore will start noticeably squashing the sound, leading to audible artifacts. Attack: a slow attack around 10ms will help preserve transients. Release: a quick release of around 0.8-1ms Threshold: 6-9db Make-up gain: Check the output - with these settings you should be balancing it out with around 1-2dB.