X-Fi: still stereo but lost stereo effect (headphone)

This is hard to describe but after installing new drivers I still have stereo sound, but when I look around in the Cessna for instance, I do not hear the engine in my left ear when I look right and I do not hear the engine in my right ear when I look left: the sound stays in the middle but it become louder! So when I look straight ahead the sound is a bit softer. So the sound IS stereo, but moving my head around doesn't have the effect it had.I do not know how to remedy this: I tried ALL various settings with my headphone, even speakersettings, everything, but nothing helps...