You'll eventually blow something if you use just a pot
wired as a variable resistor.
Make sure the pot is connected to a finite resistor to
limit the amount of audio that can be fed back into the
preamp.
Because the pot can be set low in resistance, like a little
above ZERO, there will be a setting where it will be to
close to the speaker load and send way too much signal
back in. Pfftt!
Chances are the amp will freak out with too much NFB
before that happens and you'll know something's
wrong, but at least stick a 27K-47K resistor in there
somewhere if it's a Marshall type with a presence
control and a 1K resistor if it's a Fender type.
 
Bruce