OK, good. You've eliminated one possibility at least.
You're asking the right question: "What am I missing?" Trouble is, from here at least, it seems the answer is "nothing".
However, reading the 410 manual, it does appear as if the designers were drinking too much coffee when they put the 410 together. There are lots of ways you can set some little thing incorrectly and prevent audio from appearing at the headphone outs. The monitoring scheme is very dependent on the ASIO driver and it wants for it to be ASIO 2.0. The 1.04b driver may not be what you need.
Do the setup described on page 23 to enable direct monitoring of inputs without running the signal through the computer and back and see if you hear audio. If that works, you'll know it's an issue involving the ASIO and/or some tiny thing you missed on the 410 control panel.
You are far from the first person to have issues with this damn box on OS9. You probably won't be the last.