I have exactly the same problem and the same soundcard. I have tried every possible solution/setting, i.e switching center/sub, Changing sound lvls etc. And I cant get the "bass/sub" to work properly : (
switching center/bass: makes the "bass" effect i want and it is said to have, but the downside is that i lose the center effect so speech in oblivion becomes very deep and not very loud.
not switching them: makes all sounds realistic but i dont get any sound from the sub in any games, but the center front and rear works perfecty.
All help appreciated!