Impedance is too complex a concept to give generalized answers that are still useful in a concrete application. It does indeed have to do with resistance, reactance and phase. Fortunately, for our little diystompbox problems we usually only need to concern ourselves with the sum of resistance and reactance. Resistance is the (frequency independent) resistance to a DC or AC signal (well, duh!), reactance is the frequency dependent resistance to an AC signal. Since audio is always AC, the sum of the two is also simply a frequency dependent resistance, for all we care (most of the time).

Output impedance of a circuit is the frequency dependent resistance that follows the last active component of the circuit in series until the output jack. In a real-world circuit that includes the output impedance of the last active device itself but this is often (not always!) small enough to be ignored.

Input resistance is the frequency dependent resistance to "ground" before the first active component. Beware, though, that "ground" in this case means "any and all reference voltages", including things like the virtual ground point of an inverter input. It also contains contributions from the active device itself that depend on the circuit topology and the device used. Luckily, there are equations for all reasonably common topologies and many uncommon or downright bizarre topologies to be found on the great interwebs. And most of them can be simplified dramatically without introducing too much additional errors. For example, for a non-inverting opamp, you can say "the input impedance is the total DC resistance to ground before the input", and that is wrong bit it is "correct enough" for most things where we care about impedance.

The "total impedance of a circuit" does not generally exist, especially for active circuits. So hooking a multimeter to both ends of a stompbox is not going to tell you anything about the impedance.

But for your design problem, I think the whole impedance discussion will not be super useful without much, much more other background. And learning theory is not as much fun as building a working pedal for most people, or so I'm told.

So my recommendation to you would be to try the following: You like the Red Llama (or your modded version of it), right? Take that circuit up to and including C6. Leave out the Vol pot. Add another inverter stage with a 100k pot wired as a series resistor before the inverter input and a 100k resistor in the negative feedback loop. This is now a buffered Volume control. Next add another stage with one more inverter, which you use as a 2-band Baxandall tone stack. This is the most useful tone stack topology in my personal opinion. Very flexible and easy to use. Banxanwhat? This:

http://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdfI hope I find the time to draw the whole thing for you tomorrow, but I am a bit short on time this weekend, so it may be a few more days. Unless someone beats me to it. I've been meaning to build something like that anyway, just did not find the time.

Hope that helps,

Andy