Treble disappearing while lowering a volume

Many guitars (with two volume pots) are equipped with an electronic circuit enabling them to mix in any proportion signals from pickups (even lowering to zero the volume of one of the pickup do not mute the guitar). Such a circuit cuts treble while the guitar volume is lowered (as much cut as longer is the guitar cable) and changes the guitar tone when its volume is reasonably lowered (the resistance of the pot „short circuit” the pickup). To solve this problem it is recommended to modify the circuit according to the diagram above. If after performing this step the effect of cutting treble still exists, then the reason can be long guitar cable (e.g. longer than 10 meters) or high parasitic capacitance of this cable (over 100 pF/meter).