I agree with what mick and some others are saying about taking care of a lot of things in the mix. The low end is the big deal with instruments competing, so I tend to notch out around 80 Hz in the bass guitar to make room for the kick's "oomph". It's also important like mick said to high pass some things like guitar and let the bass guitar carry the low end. Remember that there are no set rules for mixing, and if something sounds too bass heavy, then attenuate some bass from it. Just because a bass guitar is a low end instrument, it doesn't mean it can't afford to lose some bottom sometimes and get it's presence through some upper end harmonics. Hope this makes sense.