While the MiBass looks very cool, I would avoid it simply because it only does 16-bit.
24-bit might not be that important for bass, but a product with a converter that only does 16-bit makes me scratch my head.
Your description makes me think that something is wrong with either your bass' electronics or the Tascam. You shouldn't be struggling to get a decent level, even with a low-end audio interface.
However, it wouldn't be a terrible idea to add an external preamp to get a great sound. Cheap audio interfaces are most noticeably weak when it comes to their preamps.
I used to record my bass direct into a Presonus interface, and the first time I used a UA pre/compressor in front of it, WOW!
Now, I'm not suggesting you spend $1,000+ on a preamp, but putting something like an Aguilar Tone Hammer in front of the Tascam might not be a bad idea: http://www.aguilaramp.com/products_p...tonehammer.htm