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.
------------
    In addition to what DeltaBlues had to say, the p-p volume pot
can be used for all sorts of mods or boosts:
 
--mid-boost by adding a cap in parallel with the treble cap
(anything from maybe 500pF to .0022uF)
 
--tone stack defeat boost (cuts connection from 6k8 mid resistor to
ground... for a more subtle effect add a resistor across the p-p
switch contacts: 47k, 56k, or 68k). Only problem with this idea is
that you'd pull the knob out for NORMAL operation and push it in
for the boost.
 
--switch values for Ck (the cathode resistor bypass cap on the first
stage) from 0.68uf or 1.0uF to a higher value like 4.7uF or 33uF.
 
    Real amp pots with push-pull switches are worth their weight in
gold (well, maybe just tin foil).
 
--Good luck!
 
Steve Ahola
 
P.S. I don't know if DB made this clear but to BF your amp you
want to study the schematics and layout drawings for your amp and
the AB763 DR. Print out all of them and use them like a road
map...