If you go online and look at images of the way a pot works, you will see that at full volume the entire length of the resistive element is connected only to ground, and there is no resistive element in the signal path, just the wiper contact terminal and the input terminal. So at that position the pot acts essentially as a 100K grid resistor. If one wanted to use a Submissive one could replace the pot with that configuration in the input of the Crack, by disconnecting the pot and wiring the center pin of the input jack directly to the grid of the 12AU7 then putting a 100K resistor across the RCA jack from center pin to ground. But the easiest thing to do is just turn the volume all the way up on the Crack.