For this problem I have no code-example yet, but I try to describe what I want to do as best as I can, in the hope someone could give me some hints to how to go for it! 
I have a synth with some arguments, which I want to trigger using a midi-controller (connected). What I want to do is to trigger an instance of the synth with a pad of the midi-controller, and then re-set the argument values of ALWAYS THE LAST CREATED Synth-INSTANCE with a knob on the midi-controller, so that as long as the synth is sounding the parameters change (say for instance the frequency or whatever).