I don't have a US-2000 so this is just a general guideline.
Upgrade to Audacity 1.3.11 from here: http://wiki.audacityteam.org/index.php? ... tly_Builds
Connect the Tascam and switch it on.
Open Audacity and go to Edit menu > Preferences > Devices
Look in the Recording Device setting and look to see if there is an option for "multi" or something similar.
If there is, select it and set the number of channels to 16.
If there is no "multi" option, look for any other option that may relate to the Tascam and set the number of channels to 2 (stereo) and try recording with just inputs 1 and 2.
Any success so far?