I think the title says it all. I've tried recording some bass guitar but it didn't come out great, and a friend recommended that I get a DI unit. Thanks for your help.
No. You do not require a separate DI to record bass with the mobilepre.
Aslong as you are connected directly OUT from your bass to the instrument IN, you should be able to record the linelevel bass.
Im assuming you are just unsatisfied with the sound of the recorded bass? In this case, you can process ITB (in the box) to achieve desired sound. (i.e. run through amp simulation software in a DAW, etc...) If available - IMO, you will get most pleasing results by micing an amp speaker.
Also, you need to make sure you are gain staging properly. Basically, turn your bass gain knob up to the optimum level then the gain knob on your mobile pre up until you are hitting -12 on the mobpre meters.