As already mentioned, the low end in a mix comes almost exclusively from the bass guitar and kick drum- there is no advantage to EQing the master bus instead of just focusing on those two tracks. That kind of makes it like you're playing the mastering engineer, and you're fixing a mistake the mixer made...except if you're the mixer as well, it just makes no sense. If the 3dB boost you mentioned sounds about right on the master bus, then that means you should probably turn up the bass guitar volume about 3 dB, or maybe just make the same kind of low-end boost on just the bass guitar track and a bit on the kick drum.