Here’s the problem, Passive tonestacks are in every piece of your equipment, starting with the tone knob on your guitar, then almost all of your pedals and most significantly, your Amp. Passive tonestacks are reductive, this means that they shape the sound by “turning down” or altogether removing certain frequencies passing through them.
Lets start simple, probably the most common tone control utilized in effects pedals is the “BigMuff” tone control. Clearly taken from the circuit of the same name… Its very simple, two filters (High and Low pass) blended with a potentiometer. You think, “This is great! I can boost my highs or lows!” but in actuality you are reducing your highs and lows. These two filters overlap and even when the knob is at its midpoint you will lose -7.4dB unilaterally and up to -14.3dB at 1k. this is considered “Off” in our brains…
The full impact of this is illustrated below:
Keep in mind if you have three pedals in a row with passive tonestacks, they stack… This means that once your signal hits your amp (with all three at 5) it has already been diminished by 22.2dB unilaterally and 42.9dB @ 1k Hz. Now you see the importance of true bypass switching!
Related links:
How Does The Tone/Volume Control Of A Guitar Work?
How Passive Guitar Tone Controls Work