I guess the question is what are you doing with the cross-over on your AVR when you say "flip the dial to the 50hz setting on the plate amp".
I.e., with my processor set to a 250hz xover (as it is currently), I can tell you that that would cause a huge hole in my system from 50hz to 250hz or thereabouts and things would sound like cr*p (though I'm sure that possibly that particular scene would sound fine if the thump is centered somewhere below 50hz). If my avr is set to 80hz, not as much of a hole, at 50hz, likely no hole but my mains would be in trouble if I ran them down that low at reference volumes.
So what was your processor's/AVRs x-over set at?
There are so many variables that could be at play here. E.g., with my system, before fiddling quite a bit with the distance control and ultimately adding the PB13 in sealed tune back into the mix, I had a big wide dip at about 90hz or so.
The only way I could eliminate that dip was to set the crossover for the mains to 40hz as that brought the mains into play. But I wasn't willing to do that as I know pumping reference volume content at 40hz into my S2's was asking for trouble.
Thankfully the addition of the PB13 (nearfield) fixed that. E.g., see below for the 3 graphs (blue was just the dual AV15H's before playing with the distance control, purple was after optimizing distance, and green was after adding in the third sub being the PB13).