After a few months of playing my system, I have to conclude that this kind of computer music/sound card/DAC
is a low quality (as described earlier) but very convenient approach. For those who have been using iPod/iTune, they should see quality improveemnt. Viewed from an audiophile, the approach still needs a lot of improvement - long way.
Summary:
1) if a ripped a CD and play thru computer/DAC, it sounds less full body & less resolution than I play the CD on my old Sony xa9000es.
2) If I play hi-res 96khz/24 bit file, the quality is closer (but not very close) to playing SACD.
3) I listened to my friend's another known-brand DAC which reads wave files from network drive (no sound card), the quality is about the same.
Note that I listen to LP most of the time, so the approach
is ever poorer when compared to play vinyl.
Thanks for everyone's input.