Some quotes from the S1 PDF, maybe this can be instructive. Disclaimer from my part: I have no experience with Pro Logic encoding... maybe these quotes do not apply at all to that application.
"When stereo sound is broadcast via radio or TV, a significant proportion of listeners will be listening to mono. In that application, it is important that stereo mixes also be mono-compatible, i.e. that no sound in the mix becomes colored, disappears, or becomes excessively quiet..
The S1-Stereo Imager is designed such that, with material mixed on standard stereo equipment, the processing does not cause coloration in mono listening.
The mono compatibility of reprocessed stereo material can be judged using the stereo vector display. The closer the “vector” or lines corresponding to left, center and right inputs are to being vertical on the display, the more mono-compatible is the mix, whereas the closer they are to being horizontal, the quieter will be that sound position in the mono mix. In practice, mono compatibility will usually be good if all three lines are within 45 degrees of being vertical, and adequate if all three lines are within 60 degrees of being vertical.
If mono compatibility is particularly important to you, you are advised to monitor the mix via a mono reduction of the mix as well as in stereo. Many mixing desks or monitoring systems have a “mono” monitoring button."
and
"The S1 will give excellent results for adjusting the stereo effect of most kinds of standard stereo material produced in the recording studio and on conventional stereo mixing desks or their digital equivalents.
For such stereo, the only precaution needed is to ensure that the two channels are in phase. Accidental polarity reversal between the stereo channels can sometimes occur due to miswired leads or connectors in the analog domain. If stereo channels are out of phase, the S1 will not work correctly until the phase is corrected by the polarity buttons (see the section phase reversal and channel swap earlier in this manual). You may notice a polarity reversal because central stereo images are “unsharp” and bass-light in sound, or by the fact that when the output meters are in M/S mode, the level on the difference S channel is consistently larger than the M output level. With such out-of-phase stereo, you will find that center-stage sounds move in the wrong direction as you apply the rotation control. To correct this problem, click on one of the polarity buttons so that it reads “-” while the other remain at “+”.
However, for some stereo microphone techniques used for live recording, the S1 may not always work in quite the way expected. The basic advice here is simply to use your ears in judging the S1’s effect when used with such recordings. The S1 will usually work very well with recordings made with “one-point” stereo microphones in a single small housing, sound field microphones and with MS stereo microphones.
However, when the microphones in a stereo pair are spaced apart by more than a few centimeters, sounds that are not exactly center-stage will arrive at different times at the 2 microphones, and in different phases. These time and phase difference will affect the operation of the S1.
When used with stereo microphone systems where the mics are spaced apart by more than a few centimeters but less than about 1 meter, you may find that the width control does not always work well on the “phantom” sound images between the loudspeakers, and can sometimes gives such images a colored and more out-of-focus sound.
In such cases, leaving the width near a value of 1 and using only the gain, asymmetry and rotation controls may work best. You may also find it helpful to use a lower value of shuffler frequency than normal in such cases.
With stereo recordings using very widely spaced microphones (say more than 1 meter apart) as the stereo pair, you may find that rotation or asymmetry control used on their own are not effective, but that dragging them together, so that they act as a balance control, may be effective. In this case, width adjustment may again sometimes prove effective.
The S1 may not prove very effective on recordings made with a “binaural” dummy head, or similar approaches to recording such as the Schoeps “sphere” microphone or the Crown SASS system. These systems produce a very non-standard kind of stereo, and may not be improved by the S1. These systems ideally require the use of a different kind of shuffler to that incorporated into the S1 (further details on this may be found in M.A. Gerzon, “Applications of Blumlein Shuffling to Stereo Microphone Techniques”, J. Audio Eng. Soc., vol. 42 no. 6 pp. 435-453 (June 1994).). Width control may not be effective for such stereo microphones. However, although the S1 was NOT designed for use with the stereo produced by such microphone, if you like an effect it produces on them, feel free to use it."
