Stereo Tool from Flux:: has been in my tool kit ever since it came out as it is very useful for manipulation of stereo audio that isn’t as it should be, or you need to change.
Gain, Phase & Image Width
It features control of input gain and individual pan for left and right channels. A phase inverter is also available on each channel. Overall stereo pan and stereo width settings are also available to help you get the stereo image as you would like it.
Stereo Tool v3 also offers visual feedback reflecting the signal content. There is a vector scope display, as well as PPM meters for both inputs and outputs, and a phase correlation meter.
Stereo Tool v3 is available in native versions, AU, AAX and VST.