I used to own the exact same Technics decoder until about 4 years ago. I also have MythTv playing through the 990 via the USB connection.

What kind of signal is your sound card sending to the 990? Do you actually get DD/DTS out to the 990?

I don't have a sophisticated sound card, so I use USB for sound. I recommend you give that a try as well - at least you'll have a backup plan.