i have the tascam US-144 mkII, and no problem with reaper, just a f****g blue screen of death sometimes -.-v
be sure to have the latest firmware and driver, when i bought it, it didnt work with windows 7 64 bit because of this
and be sure that you have set in the right way the I/O panel in reaper, if you dont use the digital I/O, just set only the analog in reaper