First, I highly recommend reading and ABSORBING RG Keen's Technology of article. It will answer several questions you've already asked and break down the circuit in a way I'm not going to do.
The gain of the circuit is the ~8.7K total on the collector (8.2K + 470R) resistance between lugs 3 and 2 of the gain pot and the 22uF capacitor, which bypasses the 1K resistance of the pot as a whole for AC (rather than DC). Minimum gain is about 8.7x (8.7K / 1K) at minimum and whatever the transformer can spit out at maximum (when the 2-3 lugs are shorted -- this will depend on the transistor's own internal resistance).
Do a little hand math for the gain factor at each resistance value, compare it with the math for voltage gain versus dB (which is an exponential scale) and you'll see that using a linear pot requires a pretty big turn to increase the gain from, say, 8.7x to 87x (via 100 ohms of resistance between 2 and 3), which is how big you have to go to get an appreciable amount of extra gain and distortion. Where's 100R on a 1K pot? Measure it if you don't know. Now try to get to 10R (for a hypothetical 870x gain factor). You like it at 3/4 -- what's the reisstance at 3/4 on the pot? Now go grab a log pot (or reverse if you have it) and do the same measurements to see where it falls on the pot's travel.
I tried adding a resistor in front of the gain pot
This REDUCES your maximum gain, because it adds resistance between lugs 2 and 3 of the gain pot.
or maybe even diodes to ground before the output?
The output of the fuzz face (at least the classic circuit) is a voltage divider by the 8.2K between the output cap and the collector, divided by a 470R between the output cap and the power supply. The end result is you get about a half volt of output. Go ahead, plug it into a voltage divider calculator.
It sounds loud because it's a steady half volt, while we're used to a clean guitar signal maybe peaking at 1V but falling off very quickly to a much smaller voltage.
Grab your favorite diode and measure the forward voltage. That's the amount of voltage you need for the diodes to clip.
Got enough?
There are loads of posts (and again, read the Technology of) about getting more OUTPUT if you need it.
Would it be as simple as increasing the gain of the transistors ?
Well, I don't know what you're using now, so I can't say what an increase actually means. But if you want to find out, just use a couple sockets and your ears. Lots of people think that high gain transistors sound like ass in a fuzz face circuit, but they might be looking for something different.
Should i add one more stage then
Got a booster in the house anywhere? Or a breadboard? Try stuff! Experiment!
