I think eveyone has pretty much covered all the basses here (sorry, couldn't resist)
I would add and echo a few things.
Back in the days before amazing virtual drum programs that sound better than most drummers (sorry drummers), there were mountains of articles about side-chaining. this seems to have slipped by the wayside as fewer and fewer people are mixing down the individual elements of the drums to seperate tracks, prefering to use one track for the full drum mix (because it already sounds so good in isolation, right?). The problem with this approach is that the kick drums and sometimes the floor toms) are fighting for the right to those bass frequencies. The end result - a mud bath.
Try bouncing down a solo track of the kick drum only and setting up a side-chain/ducking compression to the bass so that each time the kick kicks in, the bass is ducked. this will often help with that conflict. You don't even need to keep this solo bass track in the mix, just mute it and have it working as a trigger for the side-chain.
Another culprit can be recorded acoustic guitar. we all love those bass frequencies of the acoustic guitar when played by itself, but in a mix these low bass frequencies can over-ride everything. As it is a string instrument with a big body, depending on how you mic it, the bass frequencies can be really BIG, and even imperceptible to the ear. I would run the acoustic guitars through a multiband compressor and try to see which frequencies are pumping at the low end (the left). Try putting in some compression on those lows and see if it cleans up the mix. The reason why this is so important is because, unlike the kick which plays intermitently, the acoustic guitar is often strummed, so you are swamping your mix with bass.
Just my two cents.