Compressors can help too.
Most players tend to do this. The bass player will turn himself up until he can hear his low E clearly. Problem is, when he hits other strings they will be extremely loud as the bass guitar is a dynamic instrument. So then, you've got the guitarist turning up so he can hear over the bass, and it's a never ending battle.
Like SPL said, bass takes care of the low end, guitars take care of the mids. If you're scooping your mids to get a "heavier" sound, you may be all playing low frequencies. Remember, you need the mids to cut through the mix.
Sometimes I hit notes only dogs can hear.