FF: hi freq loss with some settings

[aidoru1]

When I set my FF "fuzz knob" at middle-low values there's a hi freq loss in the output signal. Why? How could I fix it? With this control set all the way up the pedal sounds very very good.... Thank you

[petemoore]

Sounds like it may be in the 'normal' category of function for a given FF.
 All my FF's get high end grit when I turn the gain up, [also treblier W/guitar volume rolledoff] ...with gain down past ... a certain point ... and a noticable 'dulling' occurs.