"Mixed phase" mode preserves linear phase but only for high frequencies where pre-ringing artefacts are short enough to be ignored.

For "Pan" mode it affects phase response only for left-right channels difference.

For example, you have 1k bell boost. As SlickEQ M is minimum-phase, it introduces some delays/phase shifts around 1k but it's the same for both left and right channels.

Now it you pan this 1k bell boost to right, left channel EQ and right channel EQ get different responses and different delays/phase shifts. As human ear obtains pan information from both level differences and time delays, the stereo-image becomes blurred. "Mixed phase" mode compensates time delays and allows to use panning controls while keeping stereo-image stable.

On the bad side this mode introduces higher latency, higher CPU usage and less creative audio coloration.

For "Width" mode the Diff (or Side) channel equalisation affected by "width" controls gets the phase compensation described. Do like this sound or not is just a matter of taste.

If Pan and Width controls are not used (set to unity), the selection of "Mixed Phase" mode doesn't affect the sound.