I dont care what you put on your mixbus...Seriously!
I get this all the time and its a common question- "How much processing should i put on my mixbus (if any) before i send the final mix for mastering?" Short and simple answer- do whatever you want to the final mix as long as its not fully limited/compressed. It should sound how you want it as if it didn't need mastering. But DO NOT put a limiter on the mixbus. If you must it is best to send two mixes to the mastering engineer one limited and one without limiting so the engineer can reference the sound your going for. Limiting when done wrong will suck the life of the song away.
"How much headroom should i leave for the mastering stage?" -That's another question that goes along with the first question and the answer again is less complicated than most make it out to be. Again i don't care how loud it is coming in because i can reduce the level as needed, however i do care about how dynamic the mix is and there should be no internal clips. If the mix is overly compressed with no dynamics it cant be undone and makes getting the best processing very difficult. There should not be any limiting happening on the final mixbus. It is best left to the mastering engineer because the mastering engineer will need to do processing first before limiting. If the mix is already limited the outcome will sound worse, lifeless. At Life Audio Mastering we want as much "life" as we can get out of every project that we work on, but it all starts at the beginning phase of making a song.
Nothing is impossible meaning even the most compressed mixes can somewhat be rescued through use of automation and other techniques. But for best results, process the mixbuss how you want just go easy on the limiting aspect.