In pumping, could I be using the wrong term ? What's happening is during certain music passages, the high end sounds like it's fading in & out along with a little sprinkle of distortion. I thought that's what pumping meant but maybe not. Also, does the fact that unplugging the opposite channels input to the SM stops it mean anything ?
If you unplug one ch it means there is no amplitude or phase difference anymore for the unit to detect & enhance separation. Unplugging the right ch is the equivalent of center left where Lf=Lb.
I can't hear what you hear but I'd still suggest what you are experiencing is the situation of hearing the Involve separation magic doing what is does abnormally listening to just a fraction of the soundfield. Lemme put it another way. Say the recording has a lead guitar panning around in a circle or even just between left front & right front. If you listen to just left front, you will hear the guitar fade in & out as it pans around to the other speakers. That might seem like pumping.
All separation enhanced matrix decoders will produce some artifacts under certain conditions. SQ in the various Logic circuits was one of the worst. Sansui's QS Variomatrix was one of the best. Involve's updating of the original concept is even better.