I agree about the sound card issue. An external soundcard is often the best way to go for best sound.
If you are used to and like the sound of your Atoms, then most speakers that have a different sound signature will sound a bit foreign. Some B&W's tend to be brighter sounding, and thus will sound a bit harsh if you've been used to a more laid back speaker. There is no way to say: this speaker is XY% better. The B&W will have a more full bandwidth response and play louder than your smaller Atoms. Also, there are things that you can't measure: how much background detail do you hear, does the speaker separate instruments well?, what is the width and height of the image when you play it, does the speaker convey the natural sense of rhythmn well, how well does it make your feet tap?, do you find yourself fatigued after listening for long periods, does the speaker convey female voice naturally with appropriate body and lack of sibilance?, do your hear the natural texture and body of the piano notes? These are all things that separate a good speaker from a bad one and there is no way to assign a number to that.