Cool, do you pan them at all or do you keep them all panned the same?
Don't pan your bass. Keep it mono. There are no rules in music, but that is a good place to start.
To answer the other question on pre's. When you have good gear, use it. Use the DI to get a nice sound with some 3d to it. Use the gain knob to get a super light touch of harmonics and then do the same going into the preamp. If you do it right, you should notice a very rich and full bass sound that is controllable. Meaning, no harshness, no clipping, but plenty of definition from top to bottom. You have all the gear needed to get an awesome bass tone that will sit perfectly in a mix.