Can someone explain this LFO/Filter behaviour?

[ricvdb]

I'm not sure if this should be this way, maybe someone can explain. I'm pretty useless when it comes to the science behind synthesis!

When using the LFO on the filter, the higher I have the cutoff level, the less obvious the LFO effect (to the point where when the cutoff is near full, the LFO's affect isn't noticeable).  The best way I can describe it is that when the cutoff is low, the LFO's affect seems stronger and defined, and the cutoff knob acts almost like a 'release' control - so when its totally full, there is no definition from the LFO waveform.

Any explanation of the link would be appreciated, thanks!

[ricvdb]

Likewise, the cutoff treats the ENV amt similarly, in that when past 50%, the envelope is no longer noticeable

[beefinator]

Imagine the LFO and ENV depth knobs as extra hands that turn the cutoff knob (or whatever they're modulating)

The LFO is equivalent to just turning the cutoff knob back and forth a certain amount to a certain tempo.
You can try it with the cutoff knob, it can be less noticeable moving the knob around near 100% open than moving it around when it's near closed.
The filter (in lowpass mode, which I assume is what you're talking about) will filter out frequencies higher than the cutoff frequency.  If the cutoff is just moving around way above the notes that are playing, it's not going to filter out much.  (the filter cutoff maxes out at over 10kHz, maybe up to 20, I dunno.  That's not too far from the limit of hearing, so you're not going to be able to distinguish as much being filtered up there)  But when you bring the cutoff down, then it starts filtering out the lower components of the sound, which is more noticeable.

[Bruno@arturia]

The LFO is equivalent to just turning the cutoff knob back and forth a certain amount to a certain tempo.
You can try it with the cutoff knob, it can be less noticeable moving the knob around near 100% open than moving it around when it's near closed.
The filter (in lowpass mode, which I assume is what you're talking about) will filter out frequencies higher than the cutoff frequency.  If the cutoff is just moving around way above the notes that are playing, it's not going to filter out much.  (the filter cutoff maxes out at over 10kHz, maybe up to 20, I dunno.  That's not too far from the limit of hearing, so you're not going to be able to distinguish as much being filtered up there)  But when you bring the cutoff down, then it starts filtering out the lower components of the sound, which is more noticeable.

Nice explanation 
FYI max cutoff is around 17kHz when only using cutoff knob