A good tube will do it. Try a sans amp.
Most of the mixing I do is for live shows. Some days I feel like I really can't get a good sound out of a band. 9 times out of 10 it's because of the quality of the sounds coming off stage. I mix a band here called Feebo, and another called Reason Disappears. They both have one thing in common.. they keep their instruments in good condition, and they use good gear. With Feebo, I take a DI out of the Ampeg (post-pre), and with Reason Disappears, I take a DI out of his rack sans-amp.
Part of the bass sound though, is how it blends with the guitars. Crappy guitar sound will weaken the blend of bass & guitar.
Once you've got down the sound you want, remove ALL of the elements of that sound that are not important to attaining your goal. Don't add anything. You don't need any crazy harmonics processors either, but making sure the bass lines are tighly sync'ed to the kick drums will also help.
Cheers.