I took a look at some photos of the pieces you have. Please do not be insulted with any of these questions. I'm just trying to offer some help.
1) I assume you're using a mic cable(XLR to XLR) to patch from the head into the Tascam. Do you have the Volume turned up on the amp? If that DI connection is after the EQ section of the amp, then it is also after the volume/gain control. If that is too low, or off, you won't get a good level.
2) Is the volume up on your bass, and, does your bass have active electronics? If it has active electronics, when is the last time you changed the battery?
3) Does your bass work if you plug directly into the Tascam? If it doesn't, then trouble shoot that problem first.
4) Check to see if there are any FAQs on the Tascam site or user groups.
That's all I can offer without having the units in front of me.
Hope this helps!