This beast already has a shit-ton of gain, how much more gain do you want? You could try putting a booster in front of this to get an idea how much more gain you need.
By "more fat," you mean you want more bass before the distortion, right? I'd mess with that first, then see if you still need more gain. To increase the bass, increase the 100nF on the left side of the board. Try 220nF for a little more bass and 470nF for a lot more bass.
Reducing the 10K resistor or increasing the 680K resistor will increase the gain of the 2nd stage. I wouldn't go lower than 3.3K on the 10K resistor or higher than 2.2M on the 680K. Increasing the gain increases the tendency to squeal, so make sure your off-board wiring is perfect. You have been warned.
One more thing: if you want it louder, try using green, blue or white LEDs on the board.