Ye I knew.... distortion is not present until i change the equilizer to output a big Bass effect, and that's just because without that the sound is terribly annoying. Imagine while my earphone is also a Bass Booster... (actually volume is adequate but has not any bass, it's very different than convential music players)
So in summary:
1) My speaker works quite well with my integrated Realtek sound chipset.
2) My headphone woks well in every condition except with the line-out.
3) I haven't tried any other speakers to test with except a very cheap one which I also had problem with that (It sounded that it cannot amplify the signal very well - like my headphones). I thought it's the speaker so I changed that.
(actually I broke it apart )
4) I also have tested my speaker with a more advanced Realtek chipset from another PC, My integrated sound won the match. I felt that advanced Realtek from another PC cannot provide adequate volume. amazing.