Need help acheiving a stereo output.

Windows can apply audio-enhancements which should be switched-off for faithful sound reproduction.
One of these enhancements is a pseudo-surround-sound which has crossover: a little of left on right, & vice-versa.

Here’s a YouTube from Microsoft showing how to switch playback enhancements off in Windows 10 …

[ If that does not cure the crossover, check the soundcard is not applying similar enhancements ]