Am I right that the QS300 cannot be directly wired to a PC? Do I need a MIDI interface such as the USB MIDI interface Midiman Biport or Emagic MT4?

You can connect your QS300 using MIDI cable and your MIDI port on your sound card. And that is the only possible connection (excluding audio connection) as far as I know.

I hope that's what you were trying to find out.