Hi Steven, Setting you fx to the max reduces noise. (it's minor)
The available signal level has effect on the recovery side of the dumblelator.
Because there is a Local Negative Feedback(LNFB) circuit. It's acting a bit like a compressor but only with a high frequency roll-off. The high level signal peaks will roll off highs. With low signals the effect of the LNFB is minor.
So; With a pick attack the signal enters the LNFB and will roll-off highs, when a signal is decaying the highs will become more prominent because the the LNFB isn't working much.
That's why signal levels matter and also the middle knob of your dumblelator is part of that.
Hope this will help, I tried to xplain it as non-technical as possible