Hi again,
Feel a little odd about being the only one to post on my own thread, but thought I'd share the following:
TASCAM directed me to
http://www.musicxp.net/ and instructed me to follow all the tuning tips there. Following that, if I still experience any problems, the next recommended step was to uninstall the driver and put it back in from scratch.
Ironically, by the time I got back downstairs to see if they'd sent the email along, REAPER froze after hitting the play button (while still optioned for the US-1641 at default latency). If I don't start seeing much better results, I'm afraid the US-1641 is going to end up being returned. Again, any additional insights/recommendations would be greatly appreciated.
Cheers,
Alan