Increasing gain using diodes
If you have a circuit like the Jordan Bosstone or MXR Distortion+ or DOD overdrive 250 and others that have a diode pair creating the distortion at the end of a circuit, you can increase the output level of the circuit with the expense of a little distortion by putting two diodes where there were one. So instead of 2 diodes, you now have four.
This mod will increase the output level with the expense of a little distortion.
More diodes in series will also give you a wider range of clean to distorted sound but less distortion when the circuit is maxxed. Something to note is that a germanium diode "clips" at around .3V and a silicon diode "clips" around .6V so you need roughly 2 germanium diodes to equal one silicon diode. A good multimeter will allow you to measure the diode and find out what the forward voltage is. LEDs will clip even later and give you more range of clean to distortion.
Mellowing out distortion using a capacitor around diodes
You can put a capacitor in parallel around the 2 diode pair and mellow out the high end of the distortion. Increasing the capacitor value will cut out more and more of the highs.
Changing the capacitor value will alter how much highs get cut from the distortion. Start with 100pf and start working up. Try 250pf, then 470pf etc..
Asymmetrical Distortion with diodes
In the diode pair, make one diode germanium and another silicon. Alternately, put two diodes on one side and one on the other.
Essentially mismatch diodes to get this. Use a multi-meter that can measure diodes to find mismatches. Apparently some people feel this will give a more "tube-like" distortion. This is useful in both diodes to ground as shown above as well as diodes in a feedback loop like the Tube Screamer.
Increasing bass response in and out of distortion box
If the box has a small value input or output capacitor like .01uF, you can change the capacitor to a larger value such as .1uF and this will feed the unit more bass. If you change the input capacitor to a larger value, you will put more bass into the distortion circuit. If you change the output capacitor to a larger value, you will output more bass after the distortion.
Decreasing bass response in and out of the distortion box
To decrease the bass response going into the clipping part of a pedal, decrease the input capacitor's value. Usually anything smaller than .01uF will start cutting off bass. If you change the output capacitor to a smaller value, you will have the same type of clipping but will reduce the bass response at the end of the circuit. Basically a high pass filter at the end of the pedal. If you make the input capacitor very small i.e. .005uF etc... you will basically make any pedal that boosts into a treble booster!
Change the capacitor material type
A cumulative change you can make is to replace most ceramic capacitors with their film equivalents. Unfortunately film caps are usually larger than ceramics so this mainly applies to when you are going to build your own pedals or modify existing ones where space permits. Subjectively, the different between ceramic and film is an overall "smoother" sound. If you want more "grit" to your tone, try replacing film caps with their ceramic equivalents. Some people feel that cheap ceramics sound more "open" and better for distortion pedals, try these cheap ceramics and see if you can hear the difference. Try them in tone control circuits and other critical areas. Some of my best pedals used cheapo ceramics. You can also try combinations of capacitors. Take two capacitors half the value of the desired combination and make one a ceramic and the other a film to get a different tone. For example, for a .1uF cap, try .047uF ceramic and .047uF film in parallel to get a .1uF "combo" cap.
Adding a "softness" or clipping threshold control
You can put a pot in series between the signal and diode pair to adjust the clipping threshold of the diodes. This acts like a "softness" control. I saw this described by R.G. Keen.
This works well. You can also put the bounding resistor between the diodes and ground. Think of this as a "volume control" for harmonics. Remember, you can put the capacitor around the diodes as mentioned above and also mismatch the diodes if desired.
Change the diode type
Change the clipping diodes to different types to get different "shades" of distortion. LEDs give more crunch and buzz. Silicon diodes (1N4148, 1N914) are crisper (harsher?) than germanium (1N34a). Germanium, Silicon, FETs and MOSFET transistors also have diodes in them that you can use as clippers. Try different types in your circuit.
Quick and easy transistor changes
You can install transistor sockets in your stompbox so that you can mix and match transistors easily. Mouser is one place that carries them (Part No: 151-TO-18320G). Use the hFE function on your multimeter to mix and match different transistors with different gains and hear the difference.
Add a bias pot
One way to make your distortion a little more versatile is to add a bias pot. You can put a trimmer pot (sold at Radio Shack etc...) in place of any one of the bias resistors (for example on the differential distortion, no drive or bias pot is on the circuit. You can make one by replacing a bias resistor with a trimmer pot or regular pot. Any pot with a value at least as large as the original resistor value will do. Additionally, put a small value resistor in series with the replacement pot for safety. To use a pot as a variable resistor, tie one of the outer lugs to the wiper (middle lug). Now connect the middle lug to one end of where the resistor was and the other unused lug to the other remaining resistor connection.
Add a lowpass filter
A lot of distortions don't have tone controls. Here is an easy lowpass filter you can add. It will reduce your output a little but if your distortion pedal has lots of gain, this shouldn't be a big problem. If you think about it, this can go right on the lugs of the output volume pot. Use a small trimmer mounted on the lugs of the output pot along with the capacitor.
Getting more distortion out of an IC-based distorter such as the TS series
excerpt from Jack Orman's Son of Screamer
You can increase the value of the drive pot for a quick and easy mod that will give you more drive=distortion. If the pedal has two or more diodes in series, you can remove one of them to squash the signal down even more. You can also change the diodes to a different type for different "flavors" of distortion. You can also change the .05 capacitor that connects to Vr to .1uF or higher for more bass response out of the unit. To make a TS series circuit brighter, remove C4 (the .22uF to ground after the 1K resistor), this will brighten up the circuit. Another mod you could do is simply reduce the value of the 4.7K resistor R2 and this will increase the gain of the circuit. In order to keep the same frequency response, you will have to play with the value of C2.
Add a low cut filter
Here's an interesting one to try, put a low cut if your distortion has too much bass.
The pot value is 500K.
Create a band pass filter
A lowpass filter followed by a low cut filter will create a band pass filter if the lowpass frequency is higher than the low cut frequency. Changing the values of the capacitors will let different frequencies through.
Variation on lowpass filter
Make the first capacitor one value (for example: .01uF), the second capacitor another (.05uF). Make the pot fairly large ~500K. When the pot is centered, very little tone control is applied.
Big Muff Tone Circuit
I have had good success splicing this tone control in my pedals. This tone circuit goes right before the volume control.
To design your own Big Muff filter, check out Duncan's Tone Stack Calculator.