Everybody may have their different opinions here, but I'm not sure there is room for more than two bass instruments. One for melody, one for percussion. You could use multiple bass melodies, but since the ear is not very sensitive with such low frequencies, it will sound less musical and more like muddy conflicting instruments (even if the same melodies might sound good together at a higher frequency).
You're spot on with separating the instruments by EQ range. Some genres prefer to have the kick drum lower frequency and the bass or bass synth higher frequency, some genres prefer them reversed. It's probably not so much about cutting off frequencies outside of this range completely, but just choosing not to accentuate them, or choosing to tuck them away.
Rolling off lows that are lower than you need is key for getting the mud out of your bass. Do this for all higher instruments: if you've never used an EQ with a spectrum analyzer, you'd be surprised how much low frequency there is in things that sound high, even weird stuff like hi-hats and hiss. Roll each instrument off as high as you can get away with. I just recently started rolling off the lows on my bass instruments too. Of course, I start the roll-off much lower with a kick drum and bass synth, but it's still there. Power at 40-60 Hz can sound amazing, but that stacked with power at 20-30 Hz can rob the 40-60 Hz of its clarity. Preserve the frequencies you need, get rid of the rest.
Sometimes a particular kick drum sample or bass synth patch sounds muddy no matter what. I just choose a different sample or patch and move on.
And, of course, side-chain compression can help clear up the low frequencies so your kick and bass aren't competing with each other. You can overdo it for effect, or subtly do it for low frequency clarity, depending on your preference for the song.
As for providing that solid foundation of bass, I don't take the approach of adding more bass melodies that could compete. Sometimes I layer a second kick sample or a second (EQ-controlled) bass synth playing the same melody as the first. Sometimes this second synth is an octave lower or an octave higher, to add more heft or more tunefulness to the bassline. (It's weird, but a higher harmonic added through a second layer of synth can make the low synth that's already there feel much more substantial. Sometimes your ear needs higher frequency cues to fully process the lower frequency stimulus it's trying to process.)
Sometimes I use EQ to bring out the frequencies I want to add that solidity I'm after, or to cut frequencies that are masking the solidity that is already there. And sometimes I use a plugin like LoAir or Renaissance Bass to synthesize new low frequencies according to the timing and pitch of the low frequencies already present. There are lots of tools, but it just takes practice and experience to know how to use each effectively, and when one might be better suited for a song than another.
Give it a try, and trust your ears.
