Steve Kimock once detailed his process for dialing in an amp. This example is somewhat Dumble-specific, but the basics apply to anything. If your amp doesn't have the control he's referring to, skip that step.
OK, how to dial up a Dumble? I do it the same way I dial up ANY amp, which begins by listening tojust the amp, not the guitar through the amp.
1. Plug in your guitar, turn the guitar volume off.
2. Turn the amp control off, everything all the way down.
3. Get right down next to the speaker.
4. Open up your post effect master 50-75%. Listen to the amp blow through the speaker, anything?
5. Open up the front master a little bit, listen to the blow.
6. Open up the gain to 1
7. Open up the tone controls to 2 or 3
8. OK, now you should hear something. Hissshhhhpopshhh etc.
9. Rotate the input volume. Listen, you'll hear when the control starts to respond. At different places around the rotation of the pot, you'll hear the amp come on.
Most volume controls exhibit similar behavior, but the exact place they start to become active varies with the individual pot, taper, value, and circuit.
The first sweet spot is where the amp goes from nothing at all happening, to a little blow that normally starts at a pretty high frequency and then begins to pick up a little volume and low end. Take note of that orientation of the pot and remind yourself that that setting is a threshold setting, on one side one behavior, on the other side a different behavior. With whatever voltage you get from the output of your guitar, backing off on your right hand touch or digging in should give you a little change in the way the amp responds. See where this is going? We're looking for settings that exhibit this threshold or touch-sensitive behavior. That first mark on your input volume is going to be almost ridiculously low, but don't discount it yet. If something is happening there, and the amp is telling you that it is, you can exploit it in combination with the other controls.
So anyway, you get a mark around 1 or 2, or between 8 and 9 o'clock chicken head time if that's your knob. Keep going. You should hear another change in the blow coming through the speaker at around 10 o'clock chicken head. This is a real sweet spot on the Dumble, and in a very narrow range around this spot are the only good overdrive tones when you stack the gain. Much past that is just fuzz box.
Keep going!
Up around 1 or 2 o'clock will be another location on the pot where if you sweep back and forth a little you will hear the characteristic oooh-waaa of one behavior of the amp above the spot and another below. This is the territory I do the majority of me clean playing in. I can back off with my right hand and be using a wonderful clean sound or dig in and get the amp to sing, not high gain mind you, but two different sounds.
Keep going!
Past 3 o'clock on my amp the sound doesn't change much but does pick up in volume. Some amps or maybe preamp tubes will actually go into oscillation at this point, and the volume will go down, so pay attention when you get to the higher gain stuff, to check to make sure the control is doing what you think it should.
Now pick anyone of these "threshold" locations and go through the same process with the tone controls. Listen carefully for the blow to change as you work each control through its rotation by itself and in combination with the other controls. You might be surprised what you learn.
This approach will let you know when the amp is "doing something". Regardless of tube type or guitar, etc... the amp can't hide from this kind of scrutiny, and it can't lie to you either, so do it, and center your efforts in those areas where a little voltage swing from your guitar will move the amp around a little.
OK, that having been said, remember that the controls don't always do what they say they do. Volume adds bass when you turn it up, adds trebble when you turn it down, with the bright switch on. Trebble adds gain, mids don't work without bass, deep switches lose gain. Check your bass control! For a lot of stuff, you can just turn it off. Wack your low E and advance the bass control to its first threshold and leave it. That should be plenty. For what it's worth, Garcia used NO bass, bass zero on his Fender amps. Pointing to the mid-range control, he told me "That's your bass control". He was right...
Wenn nicht, weiter suchen!
(Ich zum Bleistift liebe laute Gitarren) Viele Gitarristen haben auch Ohren an den Waden. Und gut klingen tut es wenn die Hosenbeine flattern!!! 
YESSS! Aber Moment, es heisst ja "sweet spot" und nicht "bleed spot". 
