So, here's what I learned:
- Use the sound card for Dolby Digital and DTS decoding, instead of the decoder on the Logitech control console.
- Use the X-Fi instead of the speakers to upmix stereo and mono audio to 5.1
- Use whatever is easiest to control volume. This is most likely turning the volume level on the soundcard to 100% and using the control knob on the speakers to make adjustments.
- Tune the bass using the subwoofer control on the Logitech console, not the X-Fi control. Similarly, leave the THX Bass controls on the X-Fi alone and manage bass with the Logitechs.