Let's put these two together.
The English Channel has trimmer controls for drain resistors. Why? Because the FET transistors have large variations, even in the same production run. At bottom left is an instruction to adjust the trimmers for 4.5v at the drain. It is possible to select the FET's to suit fixed resistor values, the builder of the original you have traced may well have done that - or wished for good luck.
The Drain resistor alone sets the voltage gain IF the Source resistor is worth zero. But the Source resistor also helps to achieve the right bias by raising the Source voltage relative to the Gate voltage which is 0v. Note that the Gate has no positive voltage supply, it is "tied" to 0v by some resistance.
The FET channel (drain to source) is NORMALLY conducting and to be controlled, the Gate voltage needs to be some amount below the Source voltage to turn the channel towards a high resistance, a condition called "pinch off" or "cut off". The amount of voltage needed is called -Vgs in specifications. You cannot set the Gate any lower than 0V, so the Source voltage must be raised above 0v. Raising Source resistance to achieve a good bias will subtract from the available gain.
A 22uF capacitor has low impedance at guitar frequencies. In parallel with the Source resistor it performs a magic trick. DC bias is not changed since the 22uF does not pass DC. However, the Source resistor is bypassed by the AC signal through the capacitor - so gain is increased for AC signal.
Putting a resistor in series with the 22uF "bypass" capacitor - your 1k (R18) limits signal gain a bit, it lets the 22uF increase signal gain, but not by as much.