When we speak about a "digital input" that is typically coming from an external preamp and A/D (analog to digital converter), such as a UA-5, and then there is a digital signal into the recorder. At this point the recorder is just a "bit bucket", and doesn't really effect the quality of the recording. JB3's, iRiver H120's, and the Sony D-50 have that digital signal come in over a fiberoptic cable. Other recorders use a digital coax cable, frequently with an RCA jack.
Then there is "Mic in" versus "Line In", where generally speaking Mic In is more sensitive.
I'm pretty sure the Tascam doesn't have digital in at all. Strictly analog. I read some info recently on the Tascam DR-1, and it appears you can adjust the levels if you are using Mic In, but if you are using Line-in you CANNOT adjust the levels! As far as I'm concerned this is a huge miss. So if you want to use Mic In, OK, but if you want to run Line in, you better be able to adjust the levels in your preamp. This might be OK, but what about patching from SBD, or your buddies output, or running a hot mic at a loud show?