Did you have the mic/line/inst switch set to inst? This will give you a 1Meg ohm input impedance that the guitar needs to give a normal output. Any other setting and the input impedance is a lot lower, resulting in a lot of top-end loss on the guitar signal, plus a weaker overall signal.

Presumably you've been to the Tascam site and downloaded the latest drivers etc? http://tascam.com/product/us-2x2/downloads/