nshuman brings up a good point! I would also propose that what scottfeldstein is describing is an issue that plagues many bassists: the "low suck" syndrome and my own thoughts and experience:
Sometimes, a pedal is to blame. Sure, there are some popular pedals which start cutting off key lows and low mids. I eventually learned in my own experience, however, that it's often attributable more to the mistake (if you will) of dialing in 'unity gain' with your bypassed signal. Any clipping device (distortion, overdrive, fuzz, whatever) adds harmonics. Often these harmonics are midrange frequencies that our ears percieve as "more loud". Interstingly, when you start recording and watching level meters, what you thought got louder - didn't! Whammo - you're bass is lost in the mix!
Consider making your 'dirty' tone a bit louder than you think is warranted & definitely a bit more than 'unity'! Chances are, you are kicking on the grit when the musical context is ramping up in intensity and volume - your bass sound should compensate for this. The human ear is more sensitive to the mids, so don't make the mistake of adjusting your crunch tones too quiet. It makes a world of difference! You also don't want to get too muddy and lose the crunch, so beware of dialing in too much low punch.
Guitarists never have to deal with this stuff - but they also understand that their dirty tones are gonna be louder than their clean tones. There's a lesson in that, I think.