Make sure that you have the 'direct' button pushed on the channel you want to send to Cubase (output assign section). Also, apart from setting Mic1-16 on channels 1-16 (INP in your routing screen), set SLOT1-16 below each Mic input (RTN in routing screen). That way, whichever comes in, also goes out to the firewire interface - IF the 'direct' button is pushed for that channel.
Unfortunately, the Tascam forum has been down for a while, cause that was a great resource, but maybe you can find some more info here: http://tascam-dm-desks.blogspot.com/ . These desks are a tough cookie to crack at first, but once you get the hang of it, they're fantastic in every way!