Deluxe Reverb Soul Switch
 
   The "soul" switch is simply a 5K pot put in series with the negative feedback resistor. The article suggested taking out the ext speaker jack which has 2 connections. One is the speaker signal from the main jack, the other is the negative feedback wire going back to the circuit
board. Looking at the back of the switch, with the terminals up
call the terminals from left to right 1-2-3. Speaker connection
to 1, negative feedback to 2. 3 can be connected to 2 also for
increased reliability. At "0" the loop is stock. Turn it up the
negative feedback is taken way down and the amp will open up
with more gain, breakup, floppy low end (and noise.) 
 
  Another tech in town has this variation. When modding SF
master volume Fenders, he will take out the 10K mid control
and replace it with a 25K. Excellent move. He will then use the
10k as this "soul" control and mount it in the master volume
position.