R1 A Pull-down resistor to drain any DC voltage to ground from C1. Helping reduce "popping"
Any value from 1M to 2.2M is fine, no change in sound.
Yep.
C1 Sets a high pass filter along with R3. Smaller cap = less bass and a little less signal into the circuit.
Yep, sets the input high pass along with everything else up to the collector and emitter of Q1, including feedback - see below.
R2 Limits the signal in a bit as well as setting the impedance. Bigger lowers the impedance and signal.
R4 Is a feedback resistor which works along with R3, R5 and R6 to bias the transistor. Smaller = less gain
as it lets more feedback flow from collector to base.
R2, R3 and R4 set up the input and feedback for the transistor. All play multiple roles. R3 and R4 do indeed set biasing. But imagine that you replace Q1 with an inverting opamp stage, the + input being biased else where. That would make the gain of this stage be Rf/Ri = R4/R2, right? That is what is happening here, excepting for the transistor having a lower input impedance and less gain than a purpose-designed opamp. Because of the transistor's "imperfections" the gain is lower than you'd get from an opamp with the same resistors, but the operation is very similar. It's an inverting feedback stage.
C3 is a "coupling cap which forms a low pass filter with R7. Bigger = more bass (and signal) passed.
Actually, with R7 and to some extent R8, which appears as a bigger load as the pot rotates to max. At the max setting, it's almost all R8, or rather, C4 and the input impedance of the next feedback stage at Q2.
C2 across the Base and Collector rolls of some high end. Bigger + more roll off.
Correct, in the feedback around the opamp sense. This capacitor appears just like a capacitor to ground but of a value equal to the effective gain with feedback times the real capacitance; this is known as the Miller effect, and this is sometimes called a "Miller capacitor".
I believe C6 is primarily there to block DC through the diodes, which would affect bias.
This is correct; C6 and C9 also have a varying impedance, and serve to lower the gain and hence the amount of clipping at lower frequencies. Making this smaller/bigger, causes less clipping at bass/more bass to be clipped.
R5, R13 and R16 (the emitter resistors to ground) can be lowered to raise the gain, I've heard of people shunting the emitter directly to ground for a crazy - over the top fuzz. Conversely, you can reign in an unruly fuzz by raising these values.
They control the open loop gain, and as such make the approximation to an opamp more/less valid as they get smaller/bigger. With lower open loop gain, you run out of feedback gain faster and have a lower overall gain, which decreases clipping.
The collector resistors (R6, 11, 17 and 25) were components that saw a lot of different values over the years too. Anywhere from 10k - 22k. Lowering these values also tends to raise the gain.
Raising them tends to raise the open loop gain. I'm using gain in the strict sense of voltage out over voltage in, not "gain" the way guitarists say it, meaning more apparent distortion.
One last thing, R6 is there to control the lower limit of the sustain pot. without it (like on many triangle versions) you get no sound with the sustain rolled all the way back.
Yep.
It not that simple. It has few thing going on a the same time.
Absolutely correct. Most things affect other things too.
But I'd like to know the reasoning, in detail, for the values chosen for the resistors on +9v: R6, R11, R17, R24, R25.
6, 11, 17 and 25 are chosen for bias point/gain on the transistors in the open-loop sense. R24 is chosen as per the stabilized-bias setup I've typed in here many times to bias q4 properly with R21.
