Headroom isn't so you can eq. Just like everything in life running things at full throttle push boundaries. Avoiding those extremes is a good idea. Your point is valid, but if you add +3dB you no longer have headroom... Actually the goal of an eq is to remove anomalies. A perfect amp will play exactly what is put into it (ie, flat) a perfect speaker as well. Sometimes this isn't the case though. Conveniently your ear and brain aren't so good at discerning little details when they are masked in the rest of the music a peak well above the normal level though will be seriously noticeable. EQ is a tool to remove these artificial peaks or anomalies. Pretty much these are always cuts. There are exceptions based on when you've made other compromises, but you should not design a system that will require boost at the same time it is impossible to design a system that doesn't need cuts. Your comment isn't that far off though as it is still a good idea to understand that and also not boost for that reason if you are running on the hairy edge. Of course those that run hairy couldn't hear the distortion anyways Yeah I knew it was off, but the EQ idea could at least give them something perhaps more tangible.
