I don't know if this would be related to the problem of ZynaAddSubFX crashing on any attempt to load a params file....

I start JACK via qJackctl, then launch Qsynth plus two instances of ZynAddSubFX, plus Rosegarden

The next step is to make the requisite Audio connections in qJackctl, so far so good.

Finally I load a piece into Rosegarden and open the "Manage Midi Devices" dialogue for those connections to be made also.

In this dialogue there is a 130:0 Synth input port to connect the GenMidi device to but not a single available port either ZynAddSubFX, making them unusable.