Alright, so I had a very nice, well balanced mix of instruments just waiting for the vocals to be added. According to the main output, nothing goes beyond -3 db, I´m trying to keep a decent head room.  
 
I processed the vocals as I always do. The meter on them shows that I´m well below minus -6 db.  However, the main output peaks all the way to 0 db the moment I add it to the rest of the instruments. I know I'm adding additional data to the sound pool with the vocals, but it seems too much of an increase over the general mix.
 
I've checked for any hidden  saturation or twisted knobs, but nothing seems to be out of place. Not even in the main output bus. Any suggestions?

Yep - pull all your faders down by 6dB
 
Have you got every track/bus routed to your Master Bus with nothing going directly to your Main Outs other than the Master Bus?

That is correct sir, only the Master Bus is going on the Main Outs.
 
It is from the MB that I´m getting the lecture actually.

If the sum of instruments mixed with vocals is too hot, I'd assign the instruments to a bus... and the vocals to another bus... then assign those to two buses to the Master Bus.  This will allow you to control the level of all instruments and all vocals separately.
 

The level of the audio tracks and every other track in the project is all relative.
 
Vocal tracks at -6db doesn't mean anything. Its alll relative to the amount of tracks you have in each project and the effects you have on the projects and the buses you have in the project.
 
Te more tracks you have in a project, the lower each track db must be. This is because the sum of 20 tracks at a level of -10dB will be more than the sum of 10 tracks at -10dB. This means the more tracks you have, the lower each track needs to be.
 
So saying the vocals are at -6db doesn't mean squat. They can be at -12db and your master bus can still be clipping.
 
Cj

marcokorreli
Alright, so I had a very nice, well balanced mix of instruments just waiting for the vocals to be added. According to the main output, nothing goes beyond -3 db, I´m trying to keep a decent head room.  
 
I processed the vocals as I always do. The meter on them shows that I´m well below minus -6 db.  However, the main output peaks all the way to 0 db the moment I add it to the rest of the instruments. I know I'm adding additional data to the sound pool with the vocals, but it seems too much of an increase over the general mix.

This shouldn't be unexpected at all. A -6dB (peak) signal added to a copy of that signal (i.e. perfectly correlated) at -3dB would be expected to hit about +1.6dB. The sum of two uncorrelated signals at these levels might fall short of 0dB, but probably not by a lot. The nature of music is that tracks tend to be at least partially correlated to the extent that peaks follow the beat and the fundamental pitches of notes may be the same in places, so it would not be at all surprising that you would hit 0dB or more in this situation.