Why? If you use the A50's, you are going to connect via optical. That means the soundcard isn't going to DO anything; the A50's control unit is going to handle the conversion to analog, and will be the device that drives audio quality, not the soundcard.
The primary driver of audio quality is the conversion from digital to analog. When you connect to any device using a digital connection, that device, not the soundcard, is handling the conversion to analog.
You still go through the soundcard driver layer, so you'll get any benifits of audio encoding (DDL/DTS-C), environmental effects, and the like. But you'll take a massive hit in quality due to the analog conversion being done by the A50's mixamp, and not the soundcard.