Alter boost feature on Guitar Amp from a fixed +6 dB to variable +0 to +16 dB gain
Sorry to disappoint you but this portion of the circuit is purely a switch interface. The audio doesn't go through it at all. Let's see how it works:

Input off


If the BOOST SW input (on the right) is left floating the base of Q3 is pulled high via the LED and R36. Q3 turns off. 
This allows R34 and R39 to pass current from the base of Q2 so it turns on.
Q2 turning on allows current through R38 so it switches the output high to almost +9V.


Input on


If the input is connected to ground LED will turn on, Q3 will turn on and Q2 will turn off. R38 will pull the BOOST output to ground.


You need to find the next portion of the circuit that uses the BOOST signal.





Figure 1. The boost stage is built around a non-inverting amplifier with gain set by the ratio of R23 and R41.


Ignoring everything else for the moment, gain, A of the circuit is given by \$ A = 1 + \frac {R23}{R41} = 1 + \frac {22k}{22k} = 2 \$.
If Q5 is turned on it pretty well bypasses R23 so the gain drops to 
\$ A = 1 + \frac {0}{R41} = 1 + \frac {0}{22k} = 1 \$.


The ratio between the two settings is 2:1 = 6 dB.

To increase the gain to up to 16 dB we need a gain of 6.3. Rearranging our gain formula we get \$ R23 = (A - 1)R41 = (6.3 - 1) 22k = 116~k\Omega \$. 
100k is the nearest standard value and when added in series with R23 will be just about right.

Lift one end of R23, connect it to one end of the pot and connect the wiper and other end to the empty resistor pad / hole. (Connecting the wiper to the otherwise disconnected end of the pot ensures that the amp will still work if the pot wiper becomes broken / dirty / contaminated.)

For online gain to deciBel converter see sengpielaudio.com.