All params of each one of the four synths can be controlled independently by MIDI CC messages. Each synth has 23 params, namely:
MIDI CC parameter numbers start with 20, so to change the cut-off frequency for the first synth you'll need to send CC number (19 + 9) = 28.
Each next synth's CC number starts where the previous one's ends, so the second synth's decay CC number is (19 + 23 + 2) = 44, and the fourth synth's pitch is (19 + 23 * 3 + 23) = 111, which is the highest MIDI CC parameter number Percussive Synth will handle.
Though usually, you don't have to memorise the actual numbers, just use the "MIDI learn" feature in your DAW - tweak your param of interest while "midi learn" is on to connect it to a control element in your DAW.
© 2022 Vadym Rostok