Hello Geert (seems I am not the only Geert on this forum),
normally, the suggestion of eXode should indeed do what you originally asked. Selecting keynumber as modulation source, filter cutoff as destination and a negative amount should result in the cutoff value getting lower the higher you go up the keyboard.
However, since it did not give the result you wanted, let me give another suggestion. Have you actually tried just using regular ‘key amount’ in a value between 1 and 63? Of course, the cutoff value will still increase as you go up the keyboard, but the relative distance between the note frequency and the cutoff frequency will decrease, thus higher notes will be filtered more than lower notes.
As suggested, you can also use the velocity sensitivity for this, just turn up velocity amount of env 1. If you now play harder, this increases the amount of env 1, which controls filter cutoff frequency.
You also asked if you can modulate the volume? Of course you can. To modulate the volume, you have to modulate the amp envelope amount. So using velocity on env 2, your volume will go up as you play harder. If you don’t want to use velocity and your playing style, try the trick eXode gave, but select env 2 amount as destination instead of cutoff frequency. (a common mistake amongst new prophet users is to modulate VCA level, and not env 2, which will give you other results)
A final note, but an important one (as I have seen a lot of fresh prophet users struggle with it at the beginning), modulation on the prophet works by addition, not multiplication. And you can’t add above 100%. Meaning that if you already have env amount set very high, velocity (or an LFO if you want to to tremolo sounds) will have little to no effect.
Same goes for the key amount suggestion: if you are already opening up the filter very high at your lower notes (because you have the cutoff knob turned all the way open or env 1 is set up to do so), key amount will not have a lot to add and you will not hear the effect. It’s all about finding the right balance between key amount and cutoff frequency.
I hope this helps you out a bit.
Geert