KSP scripting: how to set a velocity value to get a visual feedback on a custom knob

Hi, I'm a KSP newbie and probably I'm doing something really stupid... I'm using Kontakt 5.8 engine, full version, trying to develope a simple instrument.

Every time I play a note (on note event) I'd like to have a custom knob that can visually display the current velocity value in order to give the user a visual feedback of the intensity of the played note (just for fun and aesthetics :-))

It might be a custom knob that represents a speaker, so by playing a keystroke hard (high velocity) I could show a beautiful kind of animated speaker icon with different image size and aspect in accordance with the velocity value. It should be nice effect to look at, IMO.

what I can't understand is why I can make it work if I write

$VelocityStandardKnob := $EVENT_VELOCITY where $VelocityStadardKnob is the simple built-in standard knob that Kontakt offers, in that scenario when I play a note I can see the knob reflect the velocity value but if I switch from the built-in knob type and use a custom knob with custom graphics instead (you know, it's a slider in reality), then that custom knob does not reflect the velocity value when I play a note.

I have put a message instruction after the assignment and I can see that the $VElocityCustomKnob value contains the same current value of the $VelocityStandardKnob value so I am really stucked ... what I am doing wrong ?

Thanks a lot guys, for you support.