Hello Dave and welcome to the forum.
You are correct on both counts, a 'pro-sumer' grade or higher audio interface will far surpass the stock D/A chip on your PC's motherboard and the Basic version of HW only allows 2 channel (stereo) operation. The Advanced version opens up to multichannel routing.
For starters review page 14 of the Hauptwerk manual on audio interfaces. For something a little more current and what is popular for Hauptwerk, search the forum for "audio interface".
Most audio interfaces have an application that resembles an audio mixing board. This is because they take the place of a physical mixing board in a recording studio. With this application you may be able to send the same Hauptwerk stereo channels to multiple speakers. This is akin to having an AV deck that has outputs for speakers sets A, B, and maybe C.
Danny B.