There's a misprint in the UA-20 manual. Step #5 on Page 53, says,
"Set the UA-20's ADVANCE (mode select) switch to the ON position"

it should say,
"Set the UA-20's ADVANCE (mode select) switch to the *OFF* position"

Follow the rest of the steps on page 53-55 of the manual, and the unit should work.

*** If you intend to use ASIO drivers, leav ADVANCE mode On, and don't worry about using Apple System Profiler. ***