Merde alors
Something is not correct then.
If you have that much phase shift then the stereo seperation must also be very very bad.
I prefer to first know what this is causing this than to try to change it with software, meaning the sliders.
Now to find out how can be a bit more tricky.
It can be that the transmittor input which has a capacitor to ground. A wrong designed audio deviation circuit at the oscillator can also be the case. ... so many things.
Can you give us the complete setup like which soundcard, cable and how many meters, which transmittor?