As good as advertised!
Just to follow-up to my previous anonymous post. My new Xonar D2X showed up yesterday. It took all of five minutes to install, and when I booted Mythbuntu, there it was. About two minutes later, I had reconfigured MythTV to use it as the primary sound device, complete with S/PDIF pass-through for AC3 content, and BAMMM!!!! It just worked.
So if anyone was waiting for an unsolicited AMEN to your review, consider this it! And the sound quality, all I can say is WOW. This is perhaps the best sound card I've ever owned (and I did own the original SoundBlaster way back when, so I've seen a few over the years).