The price i see that system at is comparable to a proper home theatre system and receiver package which would be better and upgradeable.
The specs of the edifer system show it getting down to 42hz, the THX certified Logitech speakers get down to 35hz, for about the same price:
http/www.edifier-international.com/products/edifier-multimedia/s550-encore
http
/logitech-en-ap.custhelp.com/app/answers/detail/a_id/22927/section/troubleshoot/crid/409/lt_product_id/7929/tabs/1,3,2,4/cl/au,en
Although the edifers 2 way satellites are attractive compared to the full range driver in the logitech system, i would still go for more low end bass. The signal to noise ratio of the ediver 85db compared to logitech 95db is also a downfall for the edifer.
But like i said, its approaching the cost of a proper home theatre package you can get from pioneer, yamaha or onkyo, which will give you much more flexibility and upgradeablility in the future.
Your not killing quality by going through a digital connection, your doing the opposite, the sound is untouched by the soundcard's circuitry more or less, but you then rely on the quality of the amp and speakers your connecting it to. by the way the xonar card does support 5.1 audio not just stereo.